Water-absorbent resin granule-containing composition and production process for water-absorbent resin granule

ABSTRACT

The invention provides: a water-absorbent resin granule-containing composition with resolution of various problems, as caused by water-absorbent resin fine powders, and with high granulation strength, and with no physical property deterioration due to granulation, and, if anything, with improvement of the absorption capacity under a load by granulation; and a process for producing the above granule. A water-absorbent resin primary particle and a water-absorbent resin granule are separately surface-crosslinked and then mixed, or mixed and then surface-crosslinked. The granulation is carried out by mixing a preheated aqueous liquid and a water-absorbent resin powder at a high speed or by supplying a water-absorbent resin powder downstream of an aqueous liquid with a continuous extrusion mixer.

This application is a continuation and claims the benefit of 35 U.S.C.§120 of U.S. patent application Ser. No. 09/093,476 filed Jun. 10, 1998now U.S. Pat. No. 6,228,930.

BACKGROUND OF THE INVENTION

A. Technical Field

The present invention relates to a water-absorbent resin composition asfavorably used for sanitary materials such as paper diapers (disposablediapers), sanitary napkins, and so-called incontinence pads. Morespecifically, the invention relates to: a water-absorbent resincomposition containing a water-absorbent resin granule which is obtainedby mixing a water-absorbent resin powder with an aqueous liquid and hashigh granulation strength and high absorption capacity under a load; anda process for producing the above granule.

B. Background Art

In recent years, water-absorbent resins are widely utilized asconstituents of sanitary materials, such as paper diapers, sanitarynapkins, and so-called incontinence pads, for the purpose of allowingthe water-absorbent resins to absorb body fluids.

As to the above-mentioned water-absorbent resins, the followingmaterials are, for example, known: crosslinked matters of partiallyneutralized polyacrylic acids, hydrolysates of starch-acrylic acid graftpolymers, saponified products of vinyl acetate-acrylic ester copolymers,hydrolysates of acrylonitrile copolymers or those of acrylamidecopolymers, or crosslinked matters of these copolymers, and crosslinkedmatters of cationic monomers.

It is said that the above-mentioned water-absorbent resins should beexcellent in the following properties: the water absorption capacity,the water absorption speed, the liquid permeability, the gel strength ofhydrogel, the suction power to suck up water from a base materialcontaining an aqueous liquid, and so on, upon contact with an aqueousliquid such as a body fluid. However, relations between these propertiesdo not necessarily display positive correlations. For example, as theabsorption capacity increases, some other properties such as liquidpermeability, gel strength, and absorption speed deteriorate.

As to a method for improving the above-mentioned water-absorptionproperties of a water-absorbent resin in good balance, an art in whichthe neighborhood of the surface of the water-absorbent resin iscrosslinked is known, and various methods have been disclosed so far,and many crosslinking agents and conditions have been proposed.

For example, methods are known in each of which the following are usedas crosslinking agents: polyhydric alcohols (JP-A-58-180233 andJP-A-61-016903); polyglycidyl compounds, polyaziridine compounds,polyamine compounds, or polyisocyanate compounds (JP-A-59-189103);glyoxal (JP-A-52-117393); polyvalent metals (JP-A-51-136588,JP-A-61-257235 and JP-A-62-007745); silane coupling agents(JP-A-61-211305, JP-A-61-252212, and JP-A-61-264006); alkylenecarbonates (DE 4020780).

In addition, as to the crosslinking conditions, other methods are alsoknown in which the following are allowed to be present during acrosslinking reaction: inert inorganic powders (JP-A-60-163956 andJP-A-60-255814); specific dihydric alcohols (JP-A-01-292004); wateralong with ether compounds (JP-A-02-153903); alkylene oxide adducts ofmonohydric alcohols, or organic acid salts, or lactams (EP 555692).

On the other hand, generally, as to a water-absorbent resin, it ispreferable that the content therein of a powder with a particle diameterof not larger than 150 μm (i.e. fine powder) is as low as possible. Thefine powder clogs even in absorbent articles such as diapers andtherefore lowers the liquid permeability. In addition, there areproblems in that the fine powder is lost as dust when handled, andfurther in that the properties such as absorption capacity under a loadare difficult to improve even if the fine powder is treated by theabove-mentioned surface-crosslinking. Thus, a water-absorbent resincontaining only a small amount of fine powder is desirable.

Conventional known methods for producing the water-absorbent resincontaining only a small amount of fine powder are, for example, asfollows: (1) a method comprising adjustment of a particle size by anoptimization of the degree of polymerization or pulverization; and (2) amethod comprising classification and removal of the formed fine powderwith a sieve or a gas current (U.S. Pat. No. 4,973,632).

However, method (1) above gives a large amount of fine powder (ten andseveral percent to tens of percent) in production process steps. Inaddition, the abolition of the fine powder as produced in method (2)above results in the much lowering of yields and the disadvantage in theabolition cost.

Thus, various proposals have been made to resolve the above-mentionedproblems by granulating or regenerating the fine powder as inevitablyformed in production processes for water-absorbent resins.

For example, EP 0463388A, U.S. Pat. Nos. 4,950,692 and 4,970,267, EP0417761A, and EP 0496594A propose methods (as means other thangranulation) for regenerating the fine powder as large particles bypulverizing and then drying a gel which is formed by mixing the finepowder with water or a hydrogel. In addition, EP 0644224 proposes agranulation method comprising the step of carrying out granulation byadding an aqueous solution of a water-soluble or water-dispersiblepolymer to a water-absorbent resin in the presence of an insolubleinorganic fine powder such that the water content of the resultantgranule can fall in the range of 30 to 70% by weight. U.S. Pat. No.5,002,986, EP 0318989B, U.S. Pat. Nos. 5,248,709, 4,123,397, 4,734,478,and 5,369,148 propose methods for increasing the average particlediameter of the fine powder to some hundreds of micrometers bygranulating the fine powder alone of about 150 micrometers to some tensof micrometers or a powdery mixture thereof with larger particles byusing a binder such as an aqueous liquid in an amount of several percentto twenty and several percent of the powder.

However, it has been difficult to uniformly add an aqueous liquid to awater-absorbent resin fine powder because its absorption speed is fastdue to its large surface area. In addition, there are problems in thatthe use of an insoluble inorganic fine powder as a mixing-promotor,generally, results not only in the disadvantage of cost, but also in theformation of dust from the insoluble inorganic fine powder or in thedeterioration of the granulation strength or the physical properties.

The present inventors found that there are problems in that even ifwater-absorbent resin powders are granulated using conventionalgranulating machines or methods, excellent absorption properties asexpected cannot be maintained in final products, probably, due todestruction of granulation in conveyance steps of the water-absorbentresins or in processing steps to the final products (for example, paperdiapers).

Furthermore, the inventors found that there might been seen physicalproperty deteriorations, such as lowering of the absorption speed,increasing of water-soluble components as impurities, or lowering of theabsorption capacity under a load, as a result of regeneration of finepowders due to the above-mentioned destruction of granulation, andfurther that, on the other hand, the inherent properties of thewater-absorbent resin deteriorate when a granulation strength isincreased by increasing the amount of an aqueous liquid, which is abinder, for the purpose of avoiding the destruction of granulation.

For example, fluidized-bed type mixers (EP 0318989) or high-speedstirring type mixers (U.S. Pat. No. 5,140,076), as conventionally usedfor granulation, provide inferior results in that the amount of anaqueous liquid as added to a water-absorbent resin powder is onlyseveral % up to at most 30%, and that it is very difficult tocontinuously and stably make granulation with the amount of the additionover 30%.

Furthermore, as to the conventional granulation methods, in the casewhere the amount of the addition of the aqueous liquid is larger than30%, the mixing of the aqueous liquid and the water-absorbent resinpowder is extremely non-uniform, and the physical-property deteriorationor particle destruction occurs due to the non-uniform addition of theaqueous liquid. Thus, there is a limitation in the amount of theaddition of the aqueous liquid for improving the granulation strength.

In addition, by the present inventors' study, it was found that: mixerswith great kneading power, as conventionally used as means other thangranulation, such as shearing mixers (EP 0417761) and Nauta type mixers,relatively facilitate the addition of the aqueous liquid, but provideinferior results in that a mixture resultant from the addition of theaqueous liquid does not form a granule, but merely forms a united largemass of a gel, and that the water-absorbent resin itself is deteriorateddue to the shearing force of the mixers.

In addition, the present inventors further found that conventionalprocesses, such as a process comprising granulation after crosslinkingthe surface neighborhood of a water-absorbent resin and a processcomprising the simultaneous steps of the granulation and thesurface-crosslinking of the water-absorbent resin, inevitably involvesurface-crosslinking fracture due to the granulation, in other words,that water-absorbent resin compositions as obtained by the conventionalgranulation processes can bear only a low load of at most about 20 g/cm²because of the fracture due to the granulation and display only a lowabsorption capacity of ten and several g/g under a high load of 50g/cm².

In addition, the present inventors further found that a water-absorbentresin primary particle alone, as obtained by removing the fine powder byclassification, is not only economically disadvantageous because of theremoval of the fine powder, but also slow in water absorption speedbecause of its small surface area, and further that a granule particlealone involves complicated process steps and is inferior because offactors such as gel fracture.

SUMMARY OF THE INVENTION

A. Objects of the Invention

Thus, the present invention has been made considering theabove-mentioned prior-art problems, and has an object to provide awater-absorbent resin granule and a composition containing it withresolution of the above-mentioned various prior-art problems, as causedby the water-absorbent resin fine powder, and with high granulationstrength, and with no physical property deterioration due togranulation, and, if anything, with improvement of the absorptioncapacity under a load by granulation.

B. Disclosure of the Invention

The present inventors made investigations in order to resolve theabove-mentioned problems, as caused by the water-absorbent resin finepowder, and to increase the granulation strength of the water-absorbentresin, and to remove the physical property deterioration which might becaused by granulation, and further to obtain a water-absorbent resingranule with physical properties better than conventional ones byaggressively using the fine powder. Consequently, the inventorsaccomplished the present invention by finding that a water-absorbentresin granule with excellent properties can be produced whenever thebelow-mentioned constitutions are satisfied in a process for producing awater-absorbent resin granule by mixing a water-absorbent resin with anaqueous liquid, and further that it is also necessary to contrive thetiming of the surface-crosslinking.

Thus, to resolve the above problems, a water-absorbent resincomposition, according to an embodiment of the present invention,comprises a product by surface-crosslinking a mixture of awater-absorbent resin primary particle and a water-absorbent resingranule.

A water-absorbent resin composition, according to another embodiment ofthe present invention, comprises a mixture of a surface-crosslinkedproduct of a water-absorbent resin primary particle and asurface-crosslinked product of a water-absorbent resin granule.

A water-absorbent resin composition, according to further anotherembodiment of the present invention, comprises a mixture of awater-absorbent resin primary particle and a water-absorbent resingranule and has an absorption capacity of at least 25 g/g for aphysiological salt solution under a load of 50 g/cm².

A process for producing a water-absorbent resin composition, accordingto an embodiment of the present invention, comprises the step of addinga crosslinking agent to a mixture of a water-absorbent resin primaryparticle and a water-absorbent resin granule, thus crosslinking thesurface neighborhood of the mixture.

A process for producing a water-absorbent resin composition, accordingto another embodiment of the present invention, comprises the step ofmixing a surface-crosslinked product of a water-absorbent resin primaryparticle and a surface-crosslinked product of a water-absorbent resingranule.

A process for producing a water-absorbent resin granule, according to anembodiment of the present invention, comprises the step of mixing awater-absorbent resin powder with a preheated aqueous liquid at a highspeed, thus obtaining a water-absorbent resin granule.

A process for producing a water-absorbent resin granule, according toanother embodiment of the present invention, comprises the steps of:supplying a water-absorbent resin powder and an aqueous liquid into acontinuous extrusion mixer having a plurality of supplying-inlets alongan arrangement of stirring-members, wherein the water-absorbent resinpowder is supplied downstream of the aqueous liquid; and mixing thewater-absorbent resin powder and the aqueous liquid in the continuousextrusion mixer, thus continuously granulating the water-absorbent resinpowder (hereinafter, this process might be referred to as “continuousgranulation process” or “process for continuously granulating”).

These and other objects and the advantages of the present invention willbe more fully apparent from the following detailed disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a measurement apparatus for the water absorptioncapacity under a load as used in the present invention.

FIG. 2 is a section illustrating an embodiment of continuous extrusionmixers as included in production apparatuses as used in granulationmethod (b) for water-absorbent resins in the present invention.

FIG. 3 is a section illustrating another embodiment of the abovecontinuous extrusion mixers.

FIG. 4 is a schematic front view of a continuous extrusion mixer as usedto continuously granulate a water-absorbent resin powder in accordancewith an embodiment of the present invention continuous granulationprocess.

FIG. 5 is a structural view illustrating the continuous extrusion mixerof FIG. 4 with a portion thereof sectional.

FIG. 6 is a section of the continuous extrusion mixer as cut along theA-A′ arrow line in FIG. 4.

FIG. 7 is a flow chart of production process steps including thegranulation of the water-absorbent resin powder.

FIG. 8 illustrates examples of granulators as used in the presentinvention process for producing a water-absorbent resin granule.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention is explained in detail.

First, an explanation is made on a process for producing awater-absorbent resin powder which is used in the present invention.

As to the water-absorbent resin powder as used in the present invention,a wide range of conventional water-absorbent resin powders areavailable, and among them, particularly, those which possess a carboxylgroup are preferable. The usable water-absorbent resin powder is apowder of conventional water-absorbent resins which are typicallyobtained by polymerizing and crosslinking hydrophilic monomerscomprising a major proportion of either or both of acrylic acid and itssalt, and form a hydrogel in water due to the absorption of as large anamount of water as 50 to 3,000 times of themselves. In addition, as tothe above-mentioned water-absorbent resins as used, the water-solublecontent therein is not larger than 25% by weight, preferably, not largerthan 15% by weight, and more preferably, not larger than 10% by weight.

Examples of the acrylic acid salt, as described above, include alkalinemetal salts, ammonium salt, and amine salts of acrylic acid. Theabove-mentioned water-absorbent resin preferably comprises acrylic acidof 10 to 40 mol % and acrylic acid salt of 60 to 90 mol % (wherein theirtotal is 100 mol %). The neutralization of acrylic acid or its polymercan be performed in the monomer form, or in the middle ofpolymerization, or after polymerization.

When the water-absorbent resin is obtained by polymerizing thehydrophilic monomers comprising a major proportion of either or both ofacrylic acid and its salt, the hydrophilic monomers are permitted toinclude monomers other than acrylic acid along with the acrylic acid orits salt.

The monomers other than acrylic acid are not especially limited, butexamples thereof include: anionic unsaturated monomers, such asmethacrylic acid, maleic acid, vinylsulfonic acid, styrenesulfonic acid,2-(meth)acrylamido-2-methylpropanesulfonic acid,2-(meth)acryloylethanesulfonic acid, and 2-(meth)acryloylpropanesulfonicacid, and their salts; nonionic unsaturated monomers containinghydrophilic groups, such as acrylamide, methacrylamide,N-ethyl(meth)acrylamide, N-n-propyl(meth)acrylamide,N-isopropyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide,2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate,methoxypolyethylene glycol (meth)acrylate, polyethylene glycolmono(meth)acrylate, vinylpyridine, N-vinylpyrrolidone,N-acryloylpiperidine, and N-acryloylpyrrolidine; and cationicunsaturated monomers such as N,N-dimethylaminoethyl (meth)acrylate,N,N-diethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl(meth)acrylate, N,N-dimethylaminopropyl (meth)acrylamide, and theirquaternary salts. These monomers may be used either alone or incombinations of at least two thereof fitly.

In the present invention, when using the monomers other than acrylicacid, the proportion of the monomers other than acrylic acid ispreferably not higher than 30 mol %, more preferably, not higher than 10mol %, of the total amount of acrylic acid and its salt as are used asthe main components.

When the hydrophilic monomers comprising a major proportion of either orboth of acrylic acid and its salt are polymerized to give thewater-absorbent resin as used in the present invention, bulkpolymerization or precipitation polymerization can be performed, but itis preferable to perform aqueous solution polymerization orreversed-phase suspension polymerization using an aqueous solution ofthe above-mentioned hydrophilic monomers in view of the good performanceachievement or the easiness in controlling the polymerization.

Incidentally, where the above-mentioned hydrophilic monomers are used inthe form of an aqueous solution thereof (hereinafter referred to as“aqueous monomer solution”), although not specifically limited, theconcentration of the monomers in the aqueous monomer solution ispreferably in the range of 10 to 70% by weight, more preferably, 20 to40% by weight. In addition, in the case of the above-mentioned aqueoussolution polymerization or reversed-phase suspension polymerization,solvents other than water may be used together with water if needarises, and the solvents used together is not specifically limited.

When the above-mentioned polymerization is initiated, the followingradical polymerization initiators, for example, can be used: potassiumpersulfate, ammonium persulfate, sodium persulfate, t-butylhydroperoxide, hydrogen peroxide, and 2,2′-azobis(2-aminodipropane)dihydrochloride.

Furthermore, it is possible to use a redox initiator as formed bycombining with the polymerization initiator a reducing agent whichaccelerates the decomposition of the initiator. Examples of the reducingagent include: sulfurous acid or (bi)sulfites such as sodium sulfite andsodium hydrogen sulfite; L-ascorbic acid (or its salts); reduciblemetals (or its salts) such as ferrous salts; and amines. However, thereducing agent is not specifically limited.

The amount of the polymerization initiator as used is not specificallylimited, but is usually in the range of 0.001 to 2 mol %, preferably0.01 to 0.5 mol %, of the monomer. In the case where the amount of theinitiator is smaller than 0.001 mol %, the amount of unreacted monomersincreases, so the amount of residual monomers in the resultantwater-absorbent resin unfavorably increases. On the other hand, in thecase where the amount of the initiator as used exceeds 2 mol %, thewater-soluble content in the resultant water-absorbent resin unfavorablyincreases, and this is also unpreferable.

In addition, instead of using the polymerization initiator, irradiationof active energy rays such as radiations, electron rays, and ultravioletrays to the reaction system may be utilized for the polymerizationreaction. Incidentally, although not specifically limited, the reactiontemperature in the above-mentioned polymerization reaction is preferablyin the range of 20 to 90° C. The reaction period of time is notspecifically limited, either, and it may be determined fitly dependingon factors such as the type of the hydrophilic monomer or polymerizationinitiator or the reaction temperature.

The water-absorbent resin, used in the present invention, may be aself-crosslinking type which does not need any crosslinking agent, butpreferable ones are those which are obtained by a copolymerization orreaction with an internal crosslinking agent having, per moleculethereof, at least two polymerizable unsaturated groups or at least tworeactive groups.

Specified examples of the internal crosslinking agent includeN,N-methylenebis(meth)acrylamide, (poly)ethylene glycol (meth)acrylate,(poly)propylene glycol di(meth)acrylate, trimethylolpropanetri(meth)acrylate, glycerol tri(meth)acrylate, glycerol acrylatemethacrylate, ethylene oxide-denatured trimethylolpropanetri(meth)acrylate, pentaerythritol hexa(meth)acrylate, triallylcyanurate, triallyl isocyanurate, triallyl phosphate, triallylamine,poly(meth)allyloxyalkanes, (poly)ethylene glycol diglycidyl ether,glycerol diglycidyl ether, ethylene glycol, polyethylene glycol,propylene glycol, glycerin, pentaerythritol, ethylenediamine, ethylenecarbonate, propylene carbonate, polyethylenimine, glycidyl(meth)acrylate.

These internal crosslinking agents may be used either alone or incombinations of at least two thereof fitly. In addition, these internalcrosslinking agents may be added to the reaction system eithercollectively or separately. In the case of using at least two internalcrosslinking agents, it is preferable to never fail to use a compoundpossessing at least two polymerizable unsaturated groups underconsideration of the absorption properties of the resultantwater-absorbent resin. The amount of the internal crosslinking agent, asused, is preferably in the range of 0.005 to 2 mol %, more preferably,0.01 to 1 mol %, of the above-mentioned monomer component. In the casewhere the amount of the above-mentioned internal crosslinking agent asused is smaller than 0.005 mol % or larger than 2 mol %, awater-absorbent resin with desired absorption properties might not beobtained.

When a crosslinked structure is introduced into the water-absorbentresin using the internal crosslinking agent, the internal crosslinkingagent may be added into the reaction system during or after thepolymerization of the monomer component, or after the polymerization andthe neutralization of the monomer component.

When carrying out the polymerization, the following materials may beadded to the reaction system: inert gases such as nitrogen; foamingagents such as (hydrogen) carbonates, carbon dioxide, azo compounds, andinert organic solvents; hydrophilic polymers such as starch-cellulose,derivatives of starch-cellulose, polyvinyl alcohol, polyacrylic acid (orits salts), and crosslinked polyacrylic acid (or its salts);surface-active agents; and chain transfer agents such as hypophosphorousacid (or its salts).

In the case where the polymer as obtained by the above polymerizationreaction is gelatinous, this gelatinous polymer is dried and then, ifnecessary, pulverized, thus obtaining the water-absorbent resin powderas used in the present invention.

Next, in the present invention, the water-absorbent resin primaryparticle and the water-absorbent resin granule are obtained using theresultant water-absorbent resin powder.

The water-absorbent resin primary particle, used as one of the rawmaterials in the present invention, is a substantially ungranulatedwater-absorbent resin powder, and is a single particle or the like whichnot a little force is needed for breaking, for example, which does notbreak due to classification or conveyance operations. It is enough thatthe water-absorbent resin primary particle in the present invention isparticulate to such a degree that the object of the present inventioncan be achieved, and the size of the water-absorbent resin primaryparticle is not especially limited. If the water absorption propertiesof the resultant water-absorbent resin composition is considered, theaverage particle diameter of the water-absorbent resin primary particleis in the range of, preferably, 150 to 800 μm, more preferably, 200 to600 μm, and further preferably the water-absorbent resin primaryparticle contains substantially no particle larger than 1,000 μm. In thecase where the average particle diameter of the water-absorbent resinprimary particle is smaller than 150 μm, the liquid permeability of theresultant water-absorbent resin composition tends to be poor, and in thecase where the average particle diameter of the water-absorbent resinprimary particle is larger than 800 μm, the absorption speed tends to below. In addition, in the case where the particle diameter of thewater-absorbent resin primary particle is too large, when used forsanitary materials the resultant water-absorbent resin composition mightgive a physical feeling of something foreign to users of the sanitarymaterials. Thus, the water-absorbent resin primary particle has aparticle diameter of, preferably, 850 to 105 μm, more preferably, 850 to150 μm, particularly preferably, 710 to 150 μm.

In addition, it is enough that the water-absorbent resin granule, usedas the other raw material in the present invention, is particulate tosuch a degree that the object of the present invention can be achieved,and the size of the water-absorbent resin primary particle is notespecially limited. If the water absorption properties of the resultantwater-absorbent resin composition is considered, the average particlediameter of the granule is in the range of, preferably, 150 to 800 μm,more preferably, 200 to 600 μm, and further preferably, the granulecontains substantially no particle larger than 1,000 μm. In the casewhere the average particle diameter of the granule is smaller than 150μm, the liquid permeability of the resultant water-absorbent resincomposition tends to be poor, and in the case where the average particlediameter of the granule is larger than 800 μm, the absorption speedtends to be low. In addition, in the case where the particle diameter ofthe granule is too large, when used as an absorbing agent of sanitarymaterials the resultant water-absorbent resin composition might give aphysical feeling of something foreign to users of the sanitarymaterials.

The water-absorbent resin powder, as used as a raw material for thegranule in the present invention, may be any form of the following: onlya fine powder of a water-absorbent resin (for example, consisting ofparticles having a particle diameter of not larger than 150 μm); amixture (for example, particles having a particle diameter of not largerthan 850 μm as a whole, including particles of not larger than 150 μm)of the fine powder and larger particles there than (for example,particles having a particle diameter of not smaller than 150 μm); afine-powder-free water-absorbent resin (for example, consisting ofparticles having a particle diameter of 150 μm to 850 μm bothinclusive). Furthermore, the usable fine powder may be a classified andremoved one from mixtures in production steps, or, for the purpose ofattaining a high absorption speed, the usable fine powder may be such asobtained alone by intentional adjustment of pulverization orpolymerization conditions.

Among these water-absorbent resin powders, the fine powder of thewater-absorbent resin is preferably used as a raw material for thewater-absorbent resin granule. The average particle diameter of the finepowder is preferably in the range of 150 to 10 μm, and the content ofparticles with a particle diameter of substantially not larger than 150μm in the fine powder is preferably not lower than 70% by weight, andmore preferably, not lower than 90% by weight. As to the shape of thefine powder, from the viewpoint of the granulation strength, anirregular shape as formed by aqueous solution polymerization ispreferred to a spherical shape as formed by reversed-phase suspensionpolymerization. Furthermore, fine powders which have not yet beensubjected to surface-crosslinking treatment is more preferable.Furthermore, it is preferable that the fine powder of thewater-absorbent resin is a product from classification of thewater-absorbent resin primary particle.

Various polymers, such as polyanion (e.g., polyethylenimine) and nonion,and polyhydric alcohols, such as glycerol, can also be used asgranulation binders for obtaining the water-absorbent resin granule inthe present invention, but, in view of physical properties or safety, itis preferable that the granulation is carried out using a bindercomprising an aqueous liquid as an essential component. The method forobtaining the water-absorbent resin granule using the aqueous liquid inthe present invention is not especially limited, but examples thereofinclude tumbling granulation methods, compression type granulationmethods, stirring type granulation methods, extrusion granulationmethods, pulverization type granulation methods, fluidized-bedgranulation methods, spray drying granulation methods. Among thesegranulation methods, the stirring type ones can be used mostconveniently. The apparatus, as used to perform such methods, may beeither a continuous or batch type, each including a tower type and asideways type. Examples of the tower type continuous granulator includeSpiral Pin Mixer made by Pacific Machinery & Engineering Co., Ltd. (FIG.8(a)) and Flow Jet Mixer made by Funken Powtex (FIG. 8(b)). Examples ofthe sideways type continuous granulator include Annular Layer Mixer madeby Draiswerke GmbH. Examples of the tower type batch granulator includeHenschel mixer made by Mitsui Kozan K.K. (FIG. 8(c)) and Turbo SphereMixer made by Moritz (FIG. 8(d)). Examples of the sideways type batchgranulator include Lödige Mixer made by Gebrüder Lödige MaschinenbauGmbH (FIG. 8(e)) and Gericke Multi-Flux Mixer made by Gericke GmbH (FIG.8(f).

The amount of the aqueous liquid, as used, is preferably not smallerthan about 1 part by weight, more preferably, in the range of about 2 toabout 280 parts by weight, per 100 parts by weight of thewater-absorbent resin powder. In the case where the amount of theaqueous liquid, as used, is too small, granulation failures easilyoccur, and therefore the object of the present invention might not beachieved.

Especially, in the case where as large an amount as 80 to 280 parts byweight of the aqueous liquid is mixed to further improve the granulationstrength or the absorption capacity under a load, a granulation methodas preferably used among the above-mentioned ones in view of goodmixing-ability is either or both of (a) a method in which the aqueousliquid is preheated prior to mixing and then the granulation is carriedout, and (b) a method in which the granulation is carried out using aspecific mixer.

In the present invention, specifically, a preferable water-absorbentresin granule is either or both of (a) a granule as obtained by aprocess comprising the step of mixing a preheated aqueous liquid into awater-absorbent resin powder at a high speed, and (b) a granule asobtained by a process in which a water-absorbent resin powder and anaqueous liquid are mixed by an adding and mixing method using ahigh-speed-stirring type continuous extrusion mixer including aplurality of impellers around a rotary shaft in a fixed cylinder,wherein the mixer is operated such that the water-absorbent resin issupplied into an area where a plurality of first impellers, as shaped togenerate an extrusion thrust, are arranged, and that the aqueous liquidis supplied into an area where a plurality of second impellers, asshaped to generate an extrusion thrust smaller than that by the firstimpellers, are arranged on the discharge side of the first impellers,thus mixing the water-absorbent resin and the aqueous liquid.

The aforementioned granulation is known as one of conventional methodsfor combining a plurality of water-absorbent resin powders with eachother to form a particle, and water or the aqueous liquid is often usedas the binder in those conventional methods. However, even if high-speedagitation type mixers (preceding U.S. Pat. No. 5,002,986 and 4,734,478),specific spray continuous granulators (U.S. Pat. No. 5,369,148),fluidized beds (EP 0318989), or the like are used in those conventionalmethods, the amount of water as added was merely at most around 30 partsby weight, even including non-uniform aggregates, per 100 parts byweight of the water-absorbent resin in view of the mixing-ability ofwater. In the case where the amount of the aqueous liquid is small, theresultant granulation strength is insufficient, so the object of thepresent invention is difficult to achieve.

In addition, a method in which mixing-promotors such as insolubleinorganic powders or water-soluble polymers are used to improve themixing-ability of water in the granulation (EP 064424) still givesnon-uniform mixing, and further, the use of the mixing-promotors ratherdeteriorates the granulation strength or the physical properties.

A process in which a hydrogel is produced from a water-absorbent resinand then kneaded and pulverized in sequence was also proposed as anotherprocess not according to the process in which a particulatewater-absorbent resin granule of a plurality of particles is directlyobtained by mixing a water-absorbent resin powder with an aqueousliquid. However, such a prior art process has problems in that, forexample, in the case where a shearing mixer (EP 0417761) or a Nauta typemixer is used to mix the above-mentioned fine powder with the aqueousliquid, even the addition and mixing of water over 100 parts by weightis possible due to the strong shearing force of the mixer, but thepowder is merely united and therefore is not particulated, and further,in the case where the kneading is carried out with too strong force, thewater-absorbent resin is deteriorated due to the shearing force of suchkneading.

Accordingly, for improving the granulation strength of thewater-absorbent resin granule without deteriorating the physicalproperties thereof, it is important to set the amount of the aqueousliquid, as added, relative to the water-absorbent resin within apredetermined range and further to directly obtain a particulatewater-absorbent resin granule. Incidentally, “to directly obtain aparticulate water-absorbent resin granule” is not a process in which aunited gel is obtained by, for example, kneading and the resultant hugegel is then pulverized or finely divided, but to obtain a particulatewater-absorbent resin with a specific particle size by aggregating aplurality of water-absorbent resin powders.

In addition, there is also a proposed process in which a water-absorbentresin powder and an aqueous liquid are mixed by kneading to form anamorphous gel, and the resultant gel is then pulverized. However, such aprior art process has problems in that in the case where a shearingmixer (EP 0417761) or a Nauta type mixer (U.S. Pat. No. 4,950,692) isused to mix the water-absorbent resin fine powder with the aqueousliquid, the water-absorbent resin is deteriorated due to the strongshearing force of the mixer. Accordingly, for improving the granulationstrength or the physical properties, it is important to directly obtaina water-absorbent resin granule by making a granulation in a short timeby mixing the water-absorbent resin powder with the aqueous liquid.

Incidentally, “to directly obtain a water-absorbent resin granule” isnot a process in which a united gel mass is obtained by, for example,kneading and the resultant huge gel mass is then pulverized or finelydivided, but to obtain a particulate water-absorbent resin granule witha specific particle size by aggregating a plurality of water-absorbentresin powders.

Because the heated aqueous liquid is used, it is possible to mix awater-absorbent resin powder with an aqueous liquid homogeneouslywithout kneading them and further without using any mixing-promotorwhich causes physical property deterioration. In addition, because theheated aqueous liquid is used, a particulate aggregate with anappropriate particle size, as formed by aggregation of respectivewater-absorbent resin particles, that is, the water-absorbent resingranule favorable for the present invention, can be produced.

The formation of the granule can be confirmed by observing with anoptical microscope a fact that a plurality of respective particles areaggregated to cohere with their particle shapes kept, or by observing afact that respective particles swell as a plurality of discontinuousparticles when absorbing a liquid.

In the present invention, it is further preferable to use either or bothof (a) the foregoing method in which the aqueous liquid is preheatedprior to mixing and then the granulation is carried out, and (b) theforegoing method in which the granulation is carried out using aspecific mixer, because, for the first time, these methods can give aparticulate water-absorbent resin granule, comprising substantiallywater and the fine powder, without using any mixing-promotor, asconventionally used for granulation, and without carrying out the gelpulverization as conventionally used as a means other than granulation.

Examples of the aqueous liquid, as used for the granulation, includewater, aqueous solutions containing hydrophilic organic solvents asdescribed hereinafter, and heated water containing a small amount ofcrosslinking agents. In this case, usable crosslinking agents includesurface-crosslinking agents with types and amounts as describedhereinafter. The joint use of the crosslinking agent with the aqueousliquid sometimes make it possible to decrease the water-soluble contentor further improve the granulation strength.

Hereinafter, a further explanation is made on the method (a) above inwhich the aqueous liquid is preheated prior to mixing and then-thegranulation is carried out.

The heating temperature of the aqueous liquid is, usually, not lowerthan 40° C., preferably, not lower than 50° C., more preferably, notlower than 60° C., still more preferably, not lower than 70° C. Theupper limit thereof is not higher than the boiling point of the aqueousliquid, and it is usually not higher than 100° C. because no remarkablechange is made even above 100° C., while the boiling point may bevariously controlled by adding salts or other solvents or changingfactors such as pressure (decreasing or increasing the pressure).

Unless the aqueous liquid is heated in advance before mixing, thewater-absorbent resin granule comprising a plurality of water-absorbentresin powders is difficult to obtain by a process in which thewater-absorbent resin powder is mixed with the aqueous liquid, andconsequently, it is impossible to control the particle diameter of theresultant water-absorbent resin granule, and in addition, in the casewhere the amount of the aqueous liquid as added is large, the resultantwater-absorbent resin granule is a united large gelatinous one, and itis impossible to actually isolate and handle it as a granulated product,and furthermore, there are problems in that the water-absorbent resinitself is degraded due to cutting-off or twining of main chains ascaused by the requirement of high mixing-power or by a kneaded state ofthe resultant gelatinous mass (incidentally, such problems can otherwisebe solved by the below-mentioned granulation method (b) as well).

On the other hand, if a simple method of heating the aqueous liquid inadvance before mixing is carried out, the water-absorbent resin granulecomprising a plurality of water-absorbent resin powders can be obtainedby mixing the water-absorbent resin powder with the aqueous liquidwithout needing any special mixer or any pulverizer to pulverize theunited gelatinous substance separately. Incidentally, thewater-absorbent resin granule, as referred to in the present invention,comprises a plurality of water-absorbent resin powders and has aparticle diameter as the granule of not larger than 20 mm, preferably,in the range of 0.3 to 10 mm, and more preferably, 0.3 to 5 mm. Inaddition, the water-absorbent resin granule, as referred to in thepresent invention, is a generic name of water-containing or drygranules, and further, a product as obtained by drying thewater-absorbent resin granule might otherwise be referred to aswater-absorbent resin granule-dried matter with a water content of notmore than 10% by weight.

For obtaining the water-absorbent resin granule to accomplish the objectof the present invention, it is preferable to preheat thewater-absorbent resin powder as well as the aqueous liquid. The heatingtemperature of the water-absorbent resin powder in the present inventionis, usually, not lower than 40° C., preferably, not lower than 50° C.,and, usually, not higher than 100° C. because no remarkable change ismade even above 100° C.

In the present invention, the aqueous liquid to be mixed with thewater-absorbent resin powder is not specifically limited, but examplesof the aqueous liquid include water and aqueous liquids containingwater-soluble salts or hydrophilic organic solvents. The water contentin the aqueous liquid is preferably not less than 90% by weight, morepreferably, not less than 99% by weight, still more preferably, in therange of 99 to 100% by weight, and particularly preferably, 100% byweight, from the viewpoint of factors such as physical properties,granulation strength, efficiency, safety, and production cost.

The amount of the aqueous liquid, as used, is, usually, not smaller than1 part by weight, preferably, not smaller than 5 parts by weight, but,in view of granulation strength, preferably in the range of 80 to 280parts by weight, per 100 parts by weight of the water-absorbent resinpowder. In the case where the amount of the aqueous liquid as usedexceeds 280 parts by weight, the resultant water-absorbent resin granuleis difficult to actually handle as the granule, and further there aredisadvantages from a viewpoint of the cost for drying. On the otherhand, in the case where the amount of the aqueous liquid, as used, issmaller than 80 parts by weight, the granulation strength might beinsufficient, and the resultant final product might therefore not beable to display excellent properties, and further there is a possibilitythat any granule could not be obtained due to non-uniform mixing.

The high-speed mixing of the heated aqueous liquid with thewater-absorbent resin powder is preferable. The high-speed mixingdenotes that the period of time for completion of mixing the aqueousliquid with the water-absorbent resin powder and formation of thewater-absorbent resin granule is short. The period of time from thecontact between the aqueous liquid and the water-absorbent resin powdertill the formation of the water-absorbent resin granule, that is, themixing period of time, is short. The mixing period of time is preferablynot longer than 3 minutes, more preferably, not longer than 1 minute,and most preferably, in the range of 1 to 60 seconds.

In the case where the mixing period of time is long, it is difficult tohomogeneously mix the aqueous liquid with the water-absorbent resinpowder, resulting in a formation of a large aggregate, and consequently,it is impossible to obtain the water-absorbent resin granule which is anobject of the present invention. Furthermore, in the case where thestirring is continued for a long time after the completion of themixing, the water-absorbent resin might involve the deteriorationthereof such as an increase of the water-soluble content and a decreaseof the absorption capacity under a load.

If the above-mentioned high-speed mixing can be achieved, the mixer asused therefor is not specifically limited, but preferable ones arevessel-fixing type mixers, particularly, mechanical-stirring typemixers. Examples of thereof include Turbulizer (made by Hosokawa MikronCo., Ltd.), Lödige Mixer (made by Gebrüder Lödige Maschinenbau GmbH), amortar mixer (made by Nishi Nihon Shikenki K.K.), Henschel mixer (madeby Mitsui Kozan K.K.), Turbo Sphere Mixer (made by Moritz), and GerickeMulti-Flux Mixer (made by Gericke GmbH). The mixer as used may be eithera batch-type mixer or a continuous-type mixer.

Next, an explanation is made on the method (b) above in which thegranulation is carried out using a specific mixer.

The specific mixer, as used in the method (b), is a continuous extrusionmixer as illustrated in JP-A-09-235378, and impellers as provided tothis mixer comprise at least two kinds of impellers of different shapes,so the mixing is carried out in at least two agitation states. As aresult, the water-absorbent resin powder is mixed with the heatedaqueous liquid more efficiently, and further uniform mixing can beensured.

In addition, the plurality of impellers in the continuous extrusionmixer, as illustrated in JP-A-09-235378, are, preferably, spirallyarranged in sequence, whereby an extrusion thrust can sufficiently beensured and further the water-absorbent resin powder or the compositethereof can smoothly be extruded. In addition, preferably, a pluralityof first impellers and a plurality of second impellers are arranged insequence around a rotary shaft in the above-mentioned continuousextrusion mixer, wherein the first impellers are set on thematerial-supplying side and shaped to generate an extrusion thrust, andthe second impellers are set on the discharge side of the firstimpellers and shaped to generate an extrusion thrust smaller than thatby the first impellers. Specifically, the first impellers give thewater-absorbent resin powder and the heated aqueous liquid a sufficientextrusion thrust into the continuous extrusion mixer, and next, thesecond impellers reduce the extrusion thrust to smaller than that by thefirst impellers, so that the mixing-stirring time can sufficiently beobtained to sufficiently carry out the mixing. Therefore, thewater-absorbent resin powder can be mixed with the (heated) aqueousliquid sufficiently uniformly. In addition, the above-mentionedcontinuous extrusion mixer preferably has such a structure that thewater-absorbent resin powder is supplied and charged into an area wherethe first impellers are arranged, and that the aqueous liquid,preferably, the heated aqueous liquid, is supplied and charged into anarea where the second impellers are arranged. Specifically, thewater-absorbent resin is fed into the continuous extrusion mixer usingthe first impellers, and next, the aqueous liquid is supplied andcharged into an area where the second impellers are arranged, thusstirring and mixing the water-absorbent resin powder and the aqueousliquid at a high speed in a moment. Therefore, the water-absorbent resinpowder can be mixed with the aqueous liquid sufficiently uniformlywithout forming any “fisheye.” Furthermore, as to the shape of theabove-mentioned first impellers, the shape of a plate is preferable asthe shape to generate an extrusion thrust, and as to the shape of theabove-mentioned second impellers, the shape of a column is preferable asthe shape to reduce the extrusion thrust to smaller than that by thefirst impellers and to thereby ensure sufficient mixing and stirring.

Incidentally, in the foregoing method (b) in which the granulation iscarried out using a specific mixer, preferable other conditions (forexample, the particle diameter of the granule, the temperature of thewater-absorbent resin powder, the type and the amount of the aqueousliquid as used, and the mixing period of time) are the same as those inthe foregoing method (a) in which the aqueous liquid is preheated priorto mixing and then the granulation is carried out. Specifically, theparticle diameter is not larger than 20 mm, preferably, in the range of0.3 to 10 mm, and more preferably, 0.3 to 5 mm. The heating temperatureof the water-absorbent resin powder is, usually, not lower than 40° C.,preferably, not lower than 50° C., but, usually, not higher than 100° C.In addition, the water content in the aqueous liquid is preferably notless than 90% by weight, more preferably, not less than 99% by weight,still more preferably, in the range of 99 to 100% by weight, andparticularly preferably, 100% by weight. Furthermore, the amount of theaqueous liquid, as used, is, usually, not smaller than 1 part by weight,preferably, not smaller than 5 parts by weight, and, in view ofgranulation strength, particularly preferably in the range of 80 to 280parts by weight, per 100 parts by weight of the water-absorbent resinpowder. The mixing period of time is preferably not longer than 3minutes, more preferably, not longer than 1 minute, and most preferably,in the range of 1 to 60 seconds.

The following water-absorbent resin granules in the present inventioncan be improved with regard to their granulation strength by dryingthem: a water-absorbent resin granule as obtained in the above way,particularly, a water-absorbent resin granule as obtained by mixing 100parts by weight of the water-absorbent resin powder with 80 to 280 partsby weight of the aqueous liquid, and further a water-absorbent resingranule as obtained by either or both of (a) the foregoing method inwhich the aqueous liquid is preheated prior to mixing and then thegranulation is carried out, and (b) the foregoing method in which thegranulation is carried out using a specific mixer. If thesewater-absorbent resin granules are dried, the fine powder is united morestrongly and thereby regenerated with as high a strength as a primaryparticle.

Incidentally, examples of the aqueous liquid, as used for the foregoinggranulation in the present invention, include water and thebelow-mentioned hydrophilic organic solvents. Among them, particularly,heated water alone or heated water containing a small amount ofcrosslinking agents is preferable as the foregoing aqueous liquid,particularly, as the foregoing heated aqueous liquid. In this case,examples of usable crosslinking agents include surface-crosslinkingagents with types and amounts as described hereinafter. The joint use ofthe crosslinking agent with the aqueous liquid in this way makes itpossible to decrease the water-soluble content or further improve thegranulation strength.

The method for drying is not specifically limited, and conventionaldryers or ovens are widely used. The drying temperature is, preferably,relatively high, concretely, in the range of 110 to 300° C., morepreferably, 120 to 200° C., still more preferably, 150 to 180° C.,because the water-absorbent resin granule contracts when dried in thesetemperature ranges, resulting in formation of a strong, dry granule. Thedrying period of time is preferably not shorter than a certain period oftime, for example, in the range of 5 minutes to 10 hours, in view ofphysical properties, and, after drying, the solid content is preferablynot less than 90% by weight. Incidentally, the dry-treatment may becarried out either for only the water-absorbent resin granule, asproduced in the present invention, or for a combination of thewater-absorbent resin granule with the polymer gel which is obtained bythe above-mentioned aqueous solution polymerization or reversed-phasesuspension polymerization and has not yet been dried.

The water-absorbent resin granule-dried matter, which is obtained in theabove way, is a strong granule as contracted by drying, but its particlesize may be regulated by pulverizing, if necessary. After pulverizing,the average particle diameter of the water-absorbent resin granule-driedmatter is in the range of, preferably, 150 to 800 μm, more preferably,200 to 600 μm. In the present invention, it is preferred that awater-absorbent resin powder, of which at least 70% by weight has aparticle diameter of not larger than 150 μm (but on average, forexample, not larger than 100 μm), is granulated so as to have an averageparticle diameter of 150 to 600 μm.

It is preferable to subject the pulverization-classification product ofthe water-absorbent resin granule, as obtained in the above way, to thebelow-mentioned surface neighborhood crosslinking treatment.Specifically, it is preferable to carry out the following process steps:a water-absorbent resin granule is produced from a water-absorbent resinfine powder by the above-mentioned granulation process of the presentinvention, and then a water-absorbent resin with only a small content ofthe fine powder is produced by making a dry granule with an averageparticle diameter of 200 to 800 μm from the above-obtainedwater-absorbent resin granule, and then surface-crosslinked, thusobtaining a water-absorbent resin composition.

The following is a further explanation about the surface-crosslinking inthe present invention.

In the present invention, the ratio by weight of the water-absorbentresin primary particle to the water-absorbent resin granule in themixture of both is in the range of 98/2 to 2/98, preferably, 95/5 to40/60. If a crosslinking agent is added to the mixture having the ratioby weight between the above materials within the above-mentioned rangeto thereby further crosslink the surface neighborhood of the mixture,then the water-absorbent resin composition with high granulationstrength and high absorption capacity under a load, fitting the objectof the present invention, can be obtained.

Too high a ratio of the water-absorbent resin granule involves granuleparticle fracture and too fast water absorption speed, and thus merelyprovides unsatisfactory results, or otherwise, too low a ratio of thewater-absorbent resin granule renders the water absorption speed of thecomposition insufficient (incidentally, the particle mixture of thewater-absorbent resin primary particle and the water-absorbent resingranule is hereinafter abbreviated as “water-absorbent resin particlemixture”).

The surface-crosslinking agent, as used in the present invention, is notespecially limited if it is a crosslinking agent with a functional groupreactive upon a functional group that the water-absorbent resin has, andconventional ones in the art are favorably used. Examples thereofinclude: polyhydric alcohols such as ethylene glycol, diethylene glycol,propylene glycol, triethylene glycol, tetraethylene glycol, polyethyleneglycol, 1,3-propanediol, dipropylene glycol,2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerol,polyglycerol, 2-buten-1,4-diol, 1,4-butanediol, 1,5-pentanediol,1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanol,trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene,oxyethylene-oxypropylene block copolymers, pentaerythritol, andsorbitol; epoxy compounds such as ethylene glycol diglycidyl ether,polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether,diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether,propylene glycol diglycidyl ether, polypropylene glycol diglycidylether, and glycidol; polyamine compounds, such as ethylenediamine,diethylenetriamine, triethylenetetramine, tetraethylenepentamine,pentaethylenehexamine, and polyethylenimine, and their inorganic ororganic salts (for example, azitidium salts); polyisocyanate compoundssuch as 2,4-tolylene diisocyanate and hexamethylene diisocyanate;polyoxazoline compounds such as 1,2-ethylenebisoxazoline; alkylenecarbonate compounds such as 1,3-dioxolan-2-one,4-methyl-1,3-dioxolan-2-one, 4,5-dimethyl-1,3-dioxolan-2-one,4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl-1,3-dioxolan-2-one,4-hydroxymethyl-1,3-dioxolane-2-one, 1,3-dioxan-2-one,4-methyl-1,3-dioxan-2-one, 4,6-dimethyl-1,3-dioxan-2-one, and1,3-dioxopan-2-one; haloepoxy compounds, such as epichlorohydrin,epibromohydrin, and α-methylepichlorohydrin, and polyvalent amineaddition products thereof (e.g. Kymene (trademark) made by Hercules);polyvalent metal compounds such as hydroxides or chlorides of zinc,calcium, magnesium, aluminum, iron, and zirconium. Thesesurface-crosslinking agents may be used alone, or may be used incombinations of at least two thereof considering their reactivity. Amongthese surface-crosslinking agents, particularly preferred is at leastone compound selected from the group consisting of polyhydric alcoholcompounds, epoxy compounds, polyamine compounds and their salts, andalkylene carbonate compounds.

As is proposed in JP-A-06-184320 (U.S. Pat. No. 5,422,405), if asurface-crosslinking agent as used for the above-mentionedsurface-crosslinking can react with a carboxyl group and comprises acombination of a first surface-crosslinking agent and a secondsurface-crosslinking agent whose solubility parameters are deferent fromeach other in the case where the water-absorbent resin particle mixturepossesses a carboxyl group, then a water-absorbent resin compositionwith still more excellent absorption capacity under a load can beobtained. The above solubility parameter is a value as commonly used asa factor indicating the polarity of compounds. Values of solubilityparameters, σ (cal/cm³)^(½), of solvents, as disclosed on pages 527-539of Polymer Handbook, 3rd edition, published by WILEY INTERSCIENCE, areapplied to the above-mentioned solubility parameter in the presentinvention. In addition, values, as applied to solubility parameters ofsolvents as not disclosed on the above-mentioned pages, are led bysubstituting Hoy's cohesive energy constant, as disclosed on page 525 ofthe Polymer Handbook above, for Small's equation as disclosed on page524 of the Polymer Handbook above.

The above-mentioned first surface-crosslinking agent is preferably acompound which is reactive upon a carboxyl group and has a solubilityparameter of 12.5 (cal/cm³)^(½) or more, further preferably, 13.0(cal/cm³)^(½) or more.

The above-mentioned second surface-crosslinking agent is preferably acompound which is reactive upon a carboxyl group and has a solubilityparameter less than 12.5 (cal/cm³)^(½), more preferably, in the range of9.5 to 12.0 (cal/cm³)^(½).

The amount of the surface-crosslinking agent, as used, depends on thecompounds as used as such, or on combinations thereof, but is preferablyin the range of 0.001 to 10 parts by weight, more preferably, 0.01 to 5parts by weight, per 100 parts by weight of the solid content of thewater-absorbent resin particle mixture.

If the above-mentioned surface-crosslinking agents are used, thecrosslinking density in the surface neighborhood of the water-absorbentresin particle mixture can be increased to a higher value than thatinside. The amount of the surface-crosslinking agent, as used, largerthan 10 parts by weight is unfavorable because it is not onlyuneconomical, but also is excessive to the formation of the optimalcrosslinking structure in the water-absorbent resin composition. Inaddition, in the case where the amount of the surface-crosslinking agentas used is smaller than 0.001 parts by weight, effects for improvingperformances, such as the absorption capacity under a load, of thewater-absorbent resin composition is unfavorably difficult to obtain.

When the water-absorbent resin particle mixture is mixed with thesurface-crosslinking agent, in the present invention, water ispreferably used as the solvent. The amount of water as used depends uponfactors such as the type, particle diameter, or water content of thewater-absorbent resin particle mixture, but is preferably in the rangeof 0 to 20 parts by weight (but not including 0 parts by weight), andpreferably in the range of 0.5 to 10 parts by weight, relative to 100parts by weight of the solid content in the water-absorbent resinparticle mixture.

When the water-absorbent resin particle mixture is mixed with thesurface-crosslinking agent, a hydrophilic organic solvent (aqueousliquid) may further be used, if necessary. Examples of the hydrophilicorganic solvent include: lower alcohols such as methyl alcohol, ethylalcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutylalcohol, and t-butyl alcohol; ketones such as acetone; ethers such asdioxane, tetrahydrofuran, and methoxy(poly)ethylene glycol; amides suchas ∈-caprolactam and N,N-dimethylformamide; and sulfoxides such asdimethyl sulfoxide. The amount of the hydrophilic organic solvent asused depends upon factors such as the type, particle diameter, or watercontent of the water-absorbent resin particle mixture, but is preferablyin the range of 20 parts by weight or less, more preferably, 0.1 to 10parts by weight, relative to 100 parts by weight of the solid content inthe water-absorbent resin particle mixture.

After the water-absorbent resin particle mixture is mixed with thesurface-crosslinking agent, a heat-treatment is carried out to theresultant mixture, thus crosslinking the surface neighborhood of thewater-absorbent resin particle mixture.

That is to say, the heat-treatment is preferable for activating thecrosslinking agent in the surface neighborhood of the water-absorbentresin particle mixture if the reactivity of the crosslinking agent, thesimplicity of production devices, and the productivity are considered.The temperature of the heat-treatment is fitly determined depending onfactors such as the type of the surface-crosslinking agent as used orthe objective crosslinking density, therefore, it is not especiallylimited, but it is preferably not lower than 80° C. When the temperatureof the heat-treatment is lower than 80° C., not only the productivitydeteriorates, but also no uniform crosslinking of the surface can beaccomplished, because a long time is consumed for the heat-treatment,thus tending to result in the lowering of absorption properties under aload and in the remaining of the surface-crosslinking agent. The heatingtemperature is in the range of, preferably, 100 to 250° C., morepreferably, 120 to 210° C. The heating period of time is preferablydetermined in the range of 1 minute to 3 hours.

The heat-treatment, as described above, can be carried out withconventional dryers or ovens. Examples of the dryers include channeltype mixing dryers, rotary dryers, desk dryers, fluidized-bed dryers,gas-stream type dryers, and infrared dryers, but there is no especiallimitation thereto.

In the present invention, it is permissible that the water-absorbentresin primary particle and the water-absorbent resin granule areseparately surface-crosslinked before mixed. The foregoingsurface-crosslinking method can, as is, be applied to the method forseparately surface-crosslinking the water-absorbent resin primaryparticle and the water-absorbent resin granule.

The water-absorbent resin composition of the present invention asobtained in the above way is a novel water-absorbent resin compositioncomprising the mixture of the water-absorbent resin primary particle andthe water-absorbent resin granule and displaying high physicalproperties, for example, an absorption capacity of at least 25 g/g for aphysiological salt solution under a load of 50 g/cm².

Granules as obtained by conventional granulation processes can bear onlya low load of at most about 20 g/cm² because of the granulationfracture, but the water-absorbent resin granule as obtained in thepresent invention displays excellent absorption even under a high loadof 50 g/cm². Therefore, because of containing such a granule, thewater-absorbent resin composition of the present invention displays ahigher absorption capacity under a high load of 50 g/cm² thanconventional ones, as well as excellent absorption speed, and further isfree from the fine powder. In addition, the water-absorbent resincomposition of the present invention preferably has the followingproperties: an absorption speed of 100 seconds or less; a water-solublecontent of 15% by weight or lower, more preferably, 10% by weight orlower; a particle size distribution of 95% by weight or higher, morepreferably, 98% by weight or higher, in terms of the proportion ofparticles with a particle size of 850 to 150 μm; and a granulationfracture ratio of 10% or less. Incidentally, the measurement methods forthese physical properties are specified in the below-mentioned examplesof some preferred embodiments according to the present invention.

It is permissible to, afford various functions to the above-mentionedwater-absorbent resin composition by further adding thereto materialssuch as disinfectants, antibacterial agents, perfumes, various inorganicpowders, foaming agents, pigments, dyes, hydrophilic short fibers,fertilizers, oxidants, reductants, water, and salts. The water-absorbentresin composition of the present invention can be applied to uses ofvarious conventional water-absorbent resins, but can favorably be usedfor absorbent articles such as absorbent-matter-containing sanitarymaterials, particularly, paper diapers, sanitary napkins, andincontinence pads, taking a serious view of the following performancesas attained by the present invention: only a small amount of finepowder, a narrow particle size distribution, excellent absorptionproperties under a load, and excellent water absorption speed.

Hereinafter, an explanation is made about an embodiment of the presentinvention while referring to FIGS. 2 and 3.

As is illustrated in FIG. 2, a high-speed-stirring type continuousextrusion mixer 1, as used for the granulation method (b) and further,favorably, for adding and mixing a crosslinking agent into a mixture ofa water-absorbent resin primary particle and a water-absorbent resingranule in accordance with the present invention, has, for example, acasing 2 as a horizontally fixed cylinder.

The casing 2 has: a material-supplying inlet 3, to project a powderywater-absorbent resin into, as illustrated in the right portion of thefigure; a liquid-supplying inlet 4, to project an aqueous liquid forgranulation into, on the discharge side of the material-supplying inlet3; and a discharge outlet 5 on the left end side of the figure.

Incidentally, as is disclosed in JP-A-04-214734, the inner face of thecasing 2 preferably comprises a base material, displaying an angle of60° or more of contact with water and a thermal deformation temperatureof 70° C. or higher, as an inner tube.

In other words, it should be noted that in the case where the basematerial displays an angle less than 60° of contact with water, thewater-absorbent resin powder or the water-absorbent resin particlemixture might non-uniformly be mixed with the aqueous liquid or thecrosslinking agent, and further that in the case where the thermaldeformation temperature of the base material is lower than 70° C., thebase material cannot sufficiently bear the heat as generated during themixing. Therefore, when the base material does not satisfy the aboveconditions, it might be impossible to continue stable mixing.

Examples of the base material preferable for the inner face of thecasing 2 include: synthetic resins such as polyethylene, polypropylene,polyester, polyamide, fluororesins, polyvinyl chloride, epoxy resins,and silicone resins; and the foregoing synthetic resins as reinforced bycombining them with inorganic fillers, such as glass, graphite, bronze,and molybdenum disulfide, or with organic fillers such as polyimide.

In addition, among the above substances, particularly desired ones arefluororesins such as polyethylene tetrafluoride, polyethylenetrifluoride, polyethylene trifluorochloride, ethylenetetrafluoride-ethylene copolymers, ethylene trifluorochloride-ethylenecopolymers, propylene pentafluoride-ethylene tetrafluoride copolymers,perfluoroalkyl vinyl ether-ethylene tetrafluoride copolymers, andpolyvinyl fluoride.

On the other hand, inside the casing 2, a rotary shaft 6 to rotationallydrive with a driving motor 8 is furnished, and around the rotary shaft6, a plurality of impellers 7 . . . are furnished.

The plurality of impellers 7 . . . are spirally arranged in sequencearound the rotary shaft 6, and comprises a plurality of first impellers7 a . . . and a plurality of second impellers 7 b . . . of which theshapes are different from those of the first impellers.

The first impellers 7 a . . . are, for example, the shape of plates,such as rectangles, to generate an extrusion thrust. Incidentally, thefirst impellers 7 a . . . are not necessarily limited to the shape ofplates such as rectangles, but, as is illustrated by a continuousextrusion mixer 10 of FIG. 2, the shapes of paddle-like plates such asflippers and butterflies are also available along with the shapes ofplates with not planar, but curved faces. In addition, as is illustratedin FIG. 2, the tip edges of the first impellers 7 a . . . do not need tobe linear, but, for example, may be arched, and further, the firstimpellers 7 a . . . may, for example, have a blade edge like a chisel.

On the other hand, the second impellers 7 b . . . are, for example, theshape of columns to generate an extrusion thrust smaller than that bythe first impellers 7 a . . .

Incidentally, the second impellers 7 b . . . are not limited to theshape of columns, either, but, for example, the shapes of bars or pinsthinner than columns are also available. The shape of the tip thereofdoes not need to be planar, either, but may be spherical, for example,hemispherical.

As is illustrated in FIG. 2, the first impellers 7 a . . . , which areplate-shaped, are furnished to a portion of the rotary shaft 6 with alength of about 35% from the end of the material-supplying inlet 3wherein the entire length of the rotary shaft 6 present in the casing 2is 100% , and, on the other hand, the second impellers 7 b . . . , whichare columnar, are furnished to a portion with a length of about 65% fromthe end on the side of the discharge outlet 5.

Thus, the first impellers 7 a . . . give the water-absorbent resin andthe aqueous liquid a sufficient extrusion thrust into the continuousextrusion mixer 1 or 10, and subsequently, the second impellers 7 b . .. reduce the extrusion thrust to smaller than that by the firstimpellers 7 a . . . , so that the mixing-stirring time can sufficientlybe obtained to sufficiently carry out the mixing or reaction.

In addition, as is illustrated by the continuous extrusion mixer 10 ofFIG. 3, some of the first impellers 7 a . . . can be furnished on thefurther discharge side of the second impellers 7 b . . . , if necessary.As to the continuous extrusion mixer 10, when the entire length of therotary shaft 6 present in the casing 2 is 100% , the first impellers 7 a. . . , which are paddle-shaped, are furnished both to a portion of therotary shaft 6 with a length of about 25% from the end of thematerial-supplying inlet 3 and to a portion of the rotary shaft 6 with alength of about 25% from the end on the side of the discharge outlet 5,and further, the second impellers 7 b . . . , of which the tips arehemispherically columnar, are furnished to the central portion otherthan the above.

The furnishing of the paddle-shaped first impellers 7 a . . . on thedischarge side, as mentioned above, enhances the extrusion thrust andfavorably discharges the resultant mixing-reaction product.

Incidentally, the pitch, at which the impellers 7 . . . are arranged, ispreferably set to match with an objective uniform mixing state.

In addition, in the continuous extrusion mixer 1 or 10, according to anembodiment of the present invention, the material-supplying inlet 3 toproject a water-absorbent resin powder or the water-absorbent resinparticle mixture into is made in an area where the first impellers 7 a .. . are arranged, and the liquid-supplying inlet 4 to project an aqueousliquid or a crosslinking agent into is made in an area where the secondimpellers 7 b . . . are arranged.

Specifically, when the water-absorbent resin and the aqueous liquid aremixed, both need to entirely contact each other as momentarily aspossible. In the case where this is insufficient, so-called “fisheyes”are formed and the uniformity of the mixing is therefore damaged. Thus,in the present invention, because the water-absorbent resin is suppliedwith the first impellers 7 a . . . into the continuous extrusion mixer 1or 10, and because the high-speed stirring-mixing of the water-absorbentresin and the aqueous liquid is carried out with the second impellers 7b . . . in a moment, the water-absorbent resin and the aqueous liquidcan sufficiently uniformly be mixed and reacted with each other.

In the case where a water-absorbent resin with a carboxyl group and anaqueous liquid containing materials reactable upon the carboxyl group,such as the crosslinking agent, are mixed or reacted with each otherusing the continuous extrusion mixer 1 having the above-mentionedconstitution, the rotary shaft 6 is allowed to rotate, for example, at ahigh speed of about 500 to about 3,000 rpm, with the driving motor 8.

In this state, the water-absorbent resin is supplied from thematerial-supplying inlet 3, thus conveying the water-absorbent resininto the continuous extrusion mixer 1 by the extrusion thrust of theplate-shaped first impellers 7 a . . . as spirally arranged.

Next, if the aqueous liquid is injected from the liquid-supplying inlet4, the water-absorbent resin and the aqueous liquid containing materialssuch as the crosslinking agent are sufficiently mixed and reacted witheach other due to the second impellers 7 b . . . giving a smallextrusion thrust, and uniformly mixed, and eventually the resultantmixing-reaction product is automatically discharged from the dischargeoutlet 5.

Next, this mixing-reaction product is, for example, dried or furthersurface-crosslinked with a heating device, thus forming awater-absorbent resin granule or composition with excellent strengthproperties.

Thus, the continuous extrusion mixers 1 and 10, according to embodimentsof the present invention, have such a structure that the plurality ofimpellers 7 . . . are furnished around the rotary shaft 6 in the fixedcasing 2 to mix and react the water-absorbent resin and the aqueousliquid together, and these impellers 7 . . . comprise at least two typesof different shapes.

In conventional continuous extrusion mixers, a plurality of impellers ofthe same shape are arranged and, therefore, merely make a non-uniformstirring and an insufficient mixing.

As to the above-mentioned embodiments of the present invention, however,because the impellers 7 . . . comprise at least two types of differentshapes, the mixing is carried out in at least two agitation states. As aresult, the water-absorbent resin is efficiently mixed and reacted withthe aqueous liquid, and uniform mixing can be ensured without anyformation of “fisheyes.” Thus, the water-absorbent resin granule and thewater-absorbent resin composition, both of which make no restriction ofthe use method thereof in final products and constantly displayexcellent properties, can be provided.

In addition, because the plurality of impellers 7 . . . are spirallyarranged in sequence, the extrusion thrust can sufficiently be ensured,and further, materials such as the water-absorbent resin can smoothly beextruded.

Furthermore, the plurality of first impellers 7 a . . . and theplurality of second impellers 7 b . . . are arranged in sequence aroundthe rotary shaft 6 in the continuous extrusion mixers 1 and 10 accordingto embodiments of the present invention, wherein the first impellers 7 a. . . are set on the material-supplying side and have shapes, such asplate shapes, to generate an extrusion thrust, and the second impellers7 b . . . are set on the discharge side of the first impellers 7 a . . .and have shapes, such as columnar shapes, to generate an extrusionthrust smaller than that by the first impellers 7 a . . .

Thus, the first impellers 7 a . . . give the water-absorbent resin andthe aqueous liquid a sufficient extrusion thrust into the continuousextrusion mixers 1 and 10, and next, the second impellers 7 b . . .reduce the extrusion thrust to smaller than that by the first impellers7 a . . . , so that the mixing-stirring time can sufficiently beobtained to sufficiently carry out the mixing or reaction. Therefore,the water-absorbent resin can be mixed or reacted with the aqueousliquid sufficiently uniformly.

In addition, because the first impellers 7 a . . . are plate-shaped, theshape thereof is preferable as the shape to generate an extrusionthrust. Furthermore, because the first impellers 7 b . . . arecolumn-shaped, the shape thereof is preferable as the shape to reducethe extrusion thrust to smaller than that by the first impellers 7 a . .. and to sufficiently ensure the mixing-stirring.

Furthermore, the inner face of the casing 2 in the continuous extrusionmixers 1 and 10 substantially comprises a base material displaying anangle of about 60° or more of contact with water and a thermaldeformation temperature of about 70° C. or higher.

In other words, in the case where the base material displays an angleless than about 60° of contact with water, the water-absorbent resinmight non-uniformly be mixed with the aqueous liquid, and further in thecase where the thermal deformation temperature of the base material islower than about 70° C., the base material cannot sufficiently bear theheat as generated during the mixing, and it might therefore beimpossible to continue stable mixing. However, the embodiments of thepresent invention can avoid these problems.

Furthermore, as to the continuous extrusion mixer 10, because aplurality of first impellers 7 a . . . are further furnished on thedischarge side of the second impellers 7 b . . . , the extrusion thrustduring the discharge is sufficiently ensured, and therefore thedischarge is favorably made.

In addition, the continuous extrusion mixers 1 and 10 have such astructure that the water-absorbent resin powder is supplied and chargedinto an area where the first impellers 7 a . . . are arranged, and thatthe Aqueous liquid is supplied and charged into an area where the secondimpellers 7 b . . . are arranged.

Thus, the water-absorbent resin is supplied with the first impellers 7 a. . . into the continuous extrusion mixer 1 or 10, and subsequently, theaqueous liquid is supplied and charged into an area where the secondimpellers 7 b . . . are arranged, thus carrying out the high-speedstirring-mixing of the water-absorbent resin and the aqueous liquid withthe second impellers 7 b . . . in a moment. As a result, thewater-absorbent resin and the aqueous liquid can sufficiently uniformlybe mixed or reacted with each other.

Incidentally, the impellers 7 . . . in the above-mentioned embodimentsof the present invention comprise the first impellers 7 a . . . and thesecond impellers 7 b . . . of the shape of two types, but the presentinvention is not limited to those embodiments. For example, impellers 7. . . of further different shapes can further be furnished to enhancethe stirring efficiency more greatly.

Hereinafter, the continuous granulation process of the present inventionis explained in detail while referring to FIGS. 4 to 7.

In the continuous granulation process of the present invention, awater-absorbent resin powder and an aqueous liquid are supplied into acontinuous extrusion mixer having a plurality of supplying-inlets alongan arrangement of stirring-members, and then the water-absorbent resinpowder and the aqueous liquid are mixed in the continuous extrusionmixer, thus continuously granulating the water-absorbent resin powder.In this process, when the water-absorbent resin powder and the aqueousliquid are supplied into the continuous extrusion mixer, thewater-absorbent resin powder is supplied downstream of the aqueousliquid, so the amount of the materials as stuck into the continuousextrusion mixer can be decreased, and thus, stable granulation cancontinuously be carried out for a long term, and further a granule withexcellent granulation strength can be obtained.

The water-absorbent resin powder and the aqueous liquid, as used in thecontinuous granulation process of the present invention, are both thesame as mentioned previously.

In the aqueous liquid, water-insoluble inorganic or organic fineparticulates may be dispersed. In addition, the aqueous liquid maycontain an organic substance with a functional group reactive upon afunctional group that the water-absorbent resin has. Examples of such anorganic substance include a crosslinking agent. If the organic substanceis used, the decrease of the water-soluble component and furtherimprovement of the granulation strength can be accomplished.

In the continuous granulation process of the present invention, theratio between the water-absorbent resin powder and the aqueous liquid,as used (supplied), may fitly be set depending upon factors such ascombinations of the water-absorbent resin powder and the aqueous liquid,as used, and uses of the resultant granule, but the ratio is preferablyset such that the amount of the supplied aqueous liquid is in the rangeof 30 to 400 parts by weight (more preferably, 80 to 280 parts byweight) per 100 parts by weight of the water-absorbent resin powder.

In the case where the amount of the aqueous liquid, as used, exceeds 400parts by weight, there is no effect to improve the granulation strengthrewarding the increase in the amount of the aqueous liquid as added, andthere are disadvantages in view of the drying cost and so on. And,again, in the case where the amount of the aqueous liquid, as used,exceeds 400 parts by weight, physical properties might be deteriorated,or it might be impossible to mix the water-absorbent resin powder andthe aqueous liquid enough uniformly.

On the other hand, in the case where the amount of the aqueous liquid,as used, is smaller than 30 parts by weight, the granulation strengthmight be insufficient, and the resultant final product might thereforenot be able to display excellent properties, and further there is apossibility that any uniform granule could not be obtained due tonon-uniform mixing.

In the continuous granulation process of the present invention,similarly to the foregoing method (a), the aqueous liquid may bepreheated before mixed with the water-absorbent resin powder. Thepreheating temperature of the aqueous liquid may be the same asmentioned for the foregoing method (a).

If the aqueous liquid is preheated in the above way, then the particlediameter of the resultant granule can easily be controlled, and theresultant granule is prevented from being obtained as a united largegelatinous one, and the burden to a motor for working the continuousextrusion mixer can be lessened.

In addition, it is preferable that the water-absorbent resin powder isalso preheated before mixing. The preheating temperature of thewater-absorbent resin powder may also be the same as mentioned for theforegoing method (a).

The continuous granulation process of the present invention isparticularly preferably applied to fine powders of water-absorbentresins (e.g. such powders having a particle diameter of not larger than150 μ) among the water-absorbent resin powders.

The fine powder of the water-absorbent resin has the foregoing problems,and thus it is desired to reduce the fine powder content in thewater-absorbent resin, whereas, as to the prior arts, continuous stablegranulation of the fine powder of the water-absorbent resin wasextremely difficult because of its large surface area, and further thefine powder of the water-absorbent resin had problems of physicalproperty deterioration, granulation fracture, etc., so no effectiveprocess for granulating the fine powder has been established. However,the continuous granulation process of the present invention can beapplied suitably for granulating such a fine powder of thewater-absorbent resin.

Hereinafter, the continuous granulation process of the present inventionis explained in more detail, especially, by exemplifying granulation ofthe water-absorbent resin powder as formed in the production process forthe water-absorbent resin.

First, an explanation is hereinafter made on an example of theproduction process for the water-absorbent resin including thegranulation process of the water-absorbent resin powder while referringto FIG. 7.

As to the water-absorbent resin powder to be granulated, its detailssuch as specified examples, starting-materials (including monomers), andsynthetic processes (polymerization processes) may be the same asmentioned previously.

As is illustrated in FIG. 7, when the water-absorbent resin is produced,the starting-monomers are first supplied into a reaction vessel, where apolymerization reaction is carried out.

In the case where the polymer as obtained by the above polymerizationreaction is a hydrogel polymer, as is illustrated in FIG. 7, thishydrogel polymer is dried and, if necessary, pulverized, and thenclassified into a coarse particle, an objective particle, and a fineparticle. Among them, the coarse particle is re-pulverized andre-classified. The objective particle is, as is or aftersurface-crosslinked, finished up to a product as a water-absorbentresin. The fine particle is granulated by mixing it with the aqueousliquid, and then dried and, if necessary, pulverized orsurface-crosslinked, and thus finished up to a product. Thus, the fineparticle can be recycled via the granulation process of the presentinvention, whereby the production efficiency of the water-absorbentresin can be enhanced. In addition, if the fine particle issurface-crosslinked after granulated, then a water-absorbent resin withstill higher physical properties can be obtained.

Incidentally, the size of the objective particle is not especiallylimited, but fitly set depending upon uses of the objective particle. Inembodiments of the continuous granulation process of the presentinvention, a water-absorbent resin powder with a particle diameterlarger than a desired particle diameter of the objective particle isreferred to as “coarse particle,” and a water-absorbent resin powderwith a particle diameter smaller than a desired particle diameter of theobjective particle is referred to as “fine particle,” but, when awater-absorbent resin powder with a particle diameter of not larger than105 μm is included in the fine particle in a ratio of at least 50% byweight thereof, effects of the present invention are particularlygreatly displayed.

In addition, the water-absorbent resin powder, as used in the continuousgranulation process of the present invention, may be asurface-crosslinked one, or may be not. Thus, when the fine particle, asformed after surface-crosslinking the objective particle (mainly duringthe crushing), is granulated, the continuous granulation process of thepresent invention can also be applied. Incidentally, the coarseparticle, as obtained in the above surface-crosslinking step, is crushed(pulverized) and classified again, thereby finally classified into theobjective particle and the fine particle.

Furthermore, as is mentioned above, the fine particle may be either aclassified one from the water-absorbent resin powder containing the fineparticle in the production process for the water-absorbent resin, or anintentionally produced one by adjustment of pulverization orpolymerization conditions for the purpose of enhancing the absorptionspeed. The water-absorbent resin powder, as granulated by the continuousgranulation process of the present invention, may be any form of thefollowing: only a fine particle of a water-absorbent resin (for example,a water-absorbent resin having a particle diameter of not larger than150 μm); a mixture of the fine particle and larger particles therethan;a fine-particle-free water-absorbent resin (for example, consisting ofparticles having a particle diameter of 150 μm to 850 μm, but notincluding 150 μm). Among these water-absorbent resin powders, fineparticles which have not yet been subjected to surface-crosslinkingtreatment are preferably used. In addition, the average particlediameter of the fine particle is preferably in the range of 150 to 10μm, and the content of particles with a particle diameter ofsubstantially not larger than 150 μm in the fine particle is preferablynot lower than 70% by weight, and more preferably, not lower than 90% byweight. Furthermore, as is mentioned above, it is still more preferablethat at least 50% by weight of the water-absorbent resin powder, asgranulated by the continuous granulation process of the presentinvention, is a fine particle (fine powder) with a particle diameter ofnot larger than 105 μm. In addition, as to the shape of these fineparticles, from the viewpoint of the granulation strength, an irregularshape as formed by aqueous solution polymerization is preferred to aspherical shape as formed by reversed-phase suspension polymerization.

Details of the surface-crosslinking agent and method, as used for theabove-mentioned surface-crosslinking in the continuous granulationprocess of the present invention, may be the same as mentionedpreviously.

Hereinafter, the above process for continuously granulating awater-absorbent resin powder is explained in more detail.

In the continuous granulation process of the present invention, thecontinuous extrusion mixer, as used to granulate the water-absorbentresin powder, is not especially limited if it has a plurality ofsupplying-inlets to supply the water-absorbent resin powder and theaqueous liquid separately from each other, and further has a structureto continuously mix the water-absorbent resin powder and the aqueousliquid by stirring them while continuously discharging the resultantmixture.

Examples of such a continuous extrusion mixer include paddles mixer ofNishimura's model (made by Nishimura Kikai Seisakusho K.K.), AnnularLayer Mixer (made by Draiswerke GmbH), Spiral Pin Mixer (made by PacificMachinery & Engineering Co., Ltd.), continuous type Lödige Mixer (madeby Gebrüder Lödige Maschinenbau GmbH), and Flow Jet Mixer (made byFunken Powtex). The continuous granulation process of the presentinvention can be carried out using these continuous extrusion mixers insuch a manner that the water-absorbent resin powder is supplieddownstream of the aqueous liquid, or using these continuous extrusionmixers as remodeled such that the water-absorbent resin powder can besupplied downstream of the aqueous liquid.

As to the continuous extrusion mixer, a high-speed-stirring typecontinuous extrusion mixer 1 of FIGS. 4 and 5 is, for example, favorablyused. This continuous extrusion mixer 1, for example, comprises: acasing 2 as a horizontally fixed cylinder; a rotary shaft 11 asfurnished inside the casing 2 and rotationally driven with a drivingmotor 13; and a plurality of stirring-members (impellers) 12 . . . asfurnished around the rotary shaft 11. Incidentally, the distance betweenthe outer periphery of the rotary shaft 11 and the inner wall of thecasing 2 is preferably set out of consideration for the stirringefficiency.

In the continuous extrusion mixer 1, the stirring-members 12 may, forexample, be the shape of paddle-like plates such as flippers andbutterflies, or the shape of plates such as rectangular, circular, oval,and triangular. In addition, the stirring-members 12 may be the shape ofplates with not planar, but curved faces, and the tip edges of thestirring-members 12 may, for example, be arched. In addition, lowerparts of the stirring-members 12 may, for example, be furnished withfixing-nuts. Furthermore, the stirring-members 12 may be prismatic, or,as is illustrated by FIG. 5, some of the stirring-members 12 . . . maybe the shape not to generate an extrusion thrust, such as columnar orpin-like, if used in combinations with the plate-shaped or prismaticones as mentioned above.

If the stirring-members 12 . . . are set in a state to mix thewater-absorbent resin powder and the aqueous liquid while conveying(extruding) them, the shape or size thereof is not especially limited.In addition, the arrangement density or position of the stirring-members12 . . . is not especially limited, but it is preferable that thestirring-members 12 . . . are arranged spirally around the rotary shaft11 for the purpose of sufficiently ensuring the extrusion thrust andsmoothly extruding the materials such as the water-absorbent resinpowder.

Furthermore, it is preferable that the surfaces of the stirring-members12 . . . and the rotary shaft 11 are coated with a film of materialssuch as Teflon resins, or plated, or coated with materials such asTeflon resin tubes, for the purpose of preventing adhesive materials,comprising a mixture of the water-absorbent resin powder and the aqueousliquid, from adhering to the stirring-members 12 . . . or the rotaryshaft 11.

For the same reason, the inner face of the casing 2 is preferablyprovided with the same base material as used for the casing of theforegoing mixer of FIG. 2 or 3.

In addition, the casing 2 has a plurality of supplying-inlets 3 to 9 anda discharge outlet 10, and among them, the supplying-inlets 3, 4, 5, and6 are arranged in this order from one end portion (left end portion inFIG. 4) toward the other end portion (right end portion in FIG. 4) ofthe casing 2 in a top wall of the horizontally fixed casing 2. Inaddition, the supplying-inlets 7, 8, and 9 are arranged in this orderfrom one end portion (left end portion in FIG. 4) toward the other endportion (right end portion in FIG. 4) of the casing 2 in a side wall ofthe casing 2. Furthermore, the discharge outlet 10 is made at an endportion (right end portion in FIG. 4), opposite to the other end portionwhere the supplying-inlet 3 is made, in a bottom wall of the casing 2.In addition, the supplying-inlets 4 and 7 are set at the same distancefrom one end portion of the rotary shaft 11 in the casing 2. Similarly,the supplying-inlets 5 and 8 as well as the supplying-inlets 6 and 9 areset at the respective same distances from one end portion of the rotaryshaft 11 in the casing 2. The supplying-inlets 3 to 9 have a structurefree to be opened and closed, and those which are disused are closed,while those which are used are, for example, connected to aproportioning supply machine 14 such that the water-absorbent resinpowder and the aqueous liquid can continuously be supplied at a certainrate.

In the continuous granulation process of the present invention, when thewater-absorbent resin powder is granulated with the continuous extrusionmixer 1, the water-absorbent resin powder is supplied downstream of theaqueous liquid.

In conventional processes, when the water-absorbent resin powder and theaqueous liquid are mixed using an extrusion mixer, the water-absorbentresin powder is first projected from a powder-projecting inlet into theextrusion mixer, and the aqueous liquid is then injected from aliquid-injecting inlet as opened downstream of the powder-projectinginlet. However, such conventional processes have problems in that whenthe amount of the supplied aqueous liquid is increased to enhance thegranulation strength, the water-absorbent resin powder and the aqueousliquid are only mixed non-uniformly, and in that adhesive materials, forexample, comprising a mixture of the water-absorbent resin powder andthe aqueous liquid, adhere to stirring-members, so the continuousgranulation is difficult to carry out stably for a long term although itmight be possible for a short term.

In comparison therewith, in the continuous granulation process of thepresent invention, because the water-absorbent resin powder is supplieddownstream of the aqueous liquid, the adhesive materials are preventedfrom adhering to inner portions of the casing 2, especially, thestirring-members 12 . . . , inner portions of the water-absorbent resinpowder-supplying inlets, or the neighborhood of the discharge outlet,whereby the granulation can be carried out continuously and stably for along term. Furthermore, even when the amount of the supplied aqueousliquid is large, the water-absorbent resin powder and the aqueous liquidcan uniformly be mixed, whereby a granule with excellent granulationstrength can be obtained.

An operation to mix the water-absorbent resin powder and the aqueousliquid using the continuous extrusion mixer 1 of the above constitutionis specified as follows: In this case, the rotary shaft 11 is firstallowed to rotate, for example, at a high speed of about 500 to about3,000 rpm, with the driving motor 13. Then, in this state, the aqueousliquid is supplied from the supplying-inlet 4 or 5 into the casing 2.The aqueous liquid, as supplied into the casing 2, is stirred with thestirring-members 12 . . . , for example, as spirally formed, whileextruded toward the discharge outlet 10 by the extrusion thrust of thestirring-members 12, and then mixed with the water-absorbent resinpowder as supplied from a supplying-inlet located downstream of theabove supplying-inlet of the aqueous liquid, for example, from thesupplying-inlet 8 or 9 corresponding respectively, and the resultantmixture (i.e. hydrogel granule) is continuously discharged from thedischarge outlet 10.

In the continuous granulation process of the present invention, thus,the water-absorbent resin powder as, for example, supplied from thesupplying-inlet 8 is mixed with the aqueous liquid as supplied from asupplying-inlet located upstream of the supplying-inlet 8, for example,from the supplying-inlet 4. In the continuous granulation process of thepresent invention, therefore, when the water-absorbent resin powder isadded, the aqueous liquid is supplied upstream of the water-absorbentresin powder and then stirred with the stirring-members 12, as setbetween the supplying-inlets 4 and 8, thus forming a layer of theaqueous liquid in the casing 2.

In the continuous granulation process of the present invention,therefore, as is illustrated in FIG. 6, a water-absorbent resin powder21 as, for example, supplied from the supplying-inlet 8 comes intocontact with a layer of a aqueous liquid 22. In the continuousgranulation process of the present invention, therefore, thewater-absorbent resin powder 21 and the aqueous liquid 22 come intocontact with each other momentarily and uniformly as a whole, andimmediately begin forming a granule (hydrogel granule 24). Accordingly,the continuous granulation process of the present invention makes itpossible to efficiently mix the water-absorbent resin powder 21 and theaqueous liquid 22 without water absorption unevenness and thus canprovide a granule with excellent granulation strength.

In addition, the continuous granulation process of the present inventionhas advantages in that: because the aqueous liquid 22 already existswhere the water-absorbent resin powder 21 is supplied, there is noprior-art problem of that a large amount of the water-absorbent resinpowder or the mixture thereof with the aqueous liquid sticks to thevicinity of the liquid injection inlet and to either or both of theinner wall of the mixer and the stirring-members as formed between theliquid injection inlet and the discharge outlet. Therefore, the hydrogelgranule 24, as obtained by the continuous granulation process of thepresent invention, is extruded continuously and stably to the side ofthe discharge outlet 10 by the stirring-members 12 . . . Incidentally,the water-absorbent resin powder 21, the aqueous liquid 22, and thehydrogel granule 24 in the casing 2 are subjected to the centrifugalforce due to the rotation of the stirring-members 12 and most of thesematerials are rotated along an outer wall of the casing 2 and extrudedto the side of the discharge outlet 10.

In the continuous granulation process of the present invention, thesupply source of the water-absorbent resin powder 21 (proportioningsupply machine 14) is not especially limited, but preferably has aproportioning ability and an extrusion ability to a certain degree. Inthe continuous granulation process of the present invention, because theaqueous liquid 22 already exists where the water-absorbent resin powder21 is supplied, the supply source of the water-absorbent resin powder 21is preferably such as has a proportioning supply ability that is nothindered by the aqueous liquid 22. Examples of the supply sourcesatisfying such a demand include ACCU-RATE DRY MATERIAL FEEDERS (made byAccu-Rate Inc.) of the type that conveys materials by rotation of asingle-shaft spiral member.

In addition, in the continuous granulation process of the presentinvention, when the water-absorbent resin powder 21 is granulated withthe continuous extrusion mixer 1, the mixing of the aqueous liquid 22and the water-absorbent resin powder 21 might be carried out whilesupplying a gas 23 upstream of the aqueous liquid 22. This is for thepurpose of preventing a danger that the inside of the continuousextrusion mixer 1 might fall into a state of reduced pressure due to therotation of the rotary shaft 11 of the continuous extrusion mixer 1, andthat the water-absorbent resin powder 21, the aqueous liquid 22, or amixture thereof might therefore flow backward from the discharge outlet10 toward the supplying-inlets 3 to 9. Incidentally, in the continuousextrusion mixer 1, there is not only the above gas 23, but also, forexample, a gas as generated due to vaporization of the aqueous liquid22.

The above gas 23 is not especially limited if it is inert to the mixingof the aqueous liquid 22 and the water-absorbent resin powder 21, andpreferable examples thereof include air and a nitrogen gas. The amountof the supplied gas 23 is set to prevent the water-absorbent resinpowder 21, the aqueous liquid 22, or a mixture thereof from flowingbackward from the discharge outlet 10 toward the supplying-inlets 3 to9, namely, to maintain the internal pressure of the continuous extrusionmixer 1 within −100 to 100 mmH₂O.

When the aqueous liquid 22 is, for example, supplied from thesupplying-inlet 4, the gas 23 is supplied from the supplying-inlet 3 aslocated upstream of the supplying-inlet 4. The supplying-position of thegas 23 is not especially limited, but the clogging of thesupplying-inlet of the gas 23 can be prevented by setting thesupplying-position of the gas 23 upstream of that of the aqueous liquid22.

When the entire length of the rotary shaft 11 present in the casing 2 is100% and when the discharge outlet 10 is made at the right end of thecasing 2, the aqueous liquid 22 is preferably supplied from asupplying-inlet which is made above a portion of the rotary shaft 11with a length of about 0 to about 55% from the end portion (left end ofthe rotary shaft 11 inside the casing 2 in FIG. 4) opposite to thedischarge outlet 10.

When the entire length of the rotary shaft 11 present in the casing 2 is100% and when the discharge outlet 10 is made at the right end of thecasing 2, the water-absorbent resin powder 21 is preferably suppliedfrom a supplying-inlet which is made above a portion of the rotary shaft11 with a length of about 10 to about 80% from the end portion (left endof the rotary shaft 11 inside the casing 2 in FIG. 4) opposite to thedischarge outlet 10 (with the proviso that the water-absorbent resinpowder 21 is supplied downstream of the aqueous liquid 22).

The supplying-position of the water-absorbent resin powder 21 is notespecially limited if it is downstream of that of the aqueous liquid 22,but it is preferably 10 to 40% downstream of that of the aqueous liquid22 when the entire length of the rotary shaft 11 present in the casing 2is 100% and when the discharge outlet 10 is made at the right end of thecasing 2. If the water-absorbent resin powder 21 is supplied 10 to 40%downstream of the aqueous liquid 22, then the aqueous liquid 22 cansufficiently be diffused into the casing 2 to form a layer thereof dueto the stirring-members 12 . . . at the supplying-position of thewater-absorbent resin powder 21, so the water-absorbent resin powder 21and the aqueous liquid 22 can be brought into contact with each otherwith good efficiency and with no unevenness. And, in this case,depending upon the revolution number of the rotary shaft 11 or upon therotation speed of the end of the stirring-member 12, the distance offrom the supplying-position of the water-absorbent resin powder 21 tothe discharge outlet 10 is preferably at least 30% , more preferably, atleast 50% , of the entire length of the rotary shaft 11 such that theresidence time of the mixture of the water-absorbent resin powder 21 andthe aqueous liquid 22 in the continuous extrusion mixer 1 cansufficiently be unsured.

Incidentally, depending upon the distance of from the supplying-positionof the water-absorbent resin powder 21 to the discharge outlet 10, theaqueous liquid 22 might not sufficiently be absorbed into thewater-absorbent resin powder 21, and the resultant hydrogel granule 24therefore might contain fisheyes. However, if the continuous granulationprocess of the present invention is performed, then the contact betweenthe water-absorbent resin powder 21 and the aqueous liquid 22 is carriedout with no unevenness, so the stirring-members 12 . . . is not hinderedfrom stirring (rotating) due to the adhesion thereto of a large amountof materials such as the water-absorbent resin powder 21 or a mixturethereof with the aqueous liquid 22. In addition, even if the resultanthydrogel granule 24 contains fisheyes immediately after discharged fromthe discharge outlet 10, it thereafter becomes uniform because thewater-absorbent resin powder 21 absorbs the aqueous liquid 22 with time.

Examples of apparatuses, with which the present invention continuousgranulation process in which the water-absorbent resin powder issupplied downstream of the aqueous liquid can be performed, other thanthe above continuous extrusion mixer include: Spiral Pin Mixer made byPacific Machinery & Engineering Co., Ltd.; Flow Jet Mixer made by FunkenPowtex; and Annular Layer Mixer made by Draiswerke GmbH.

The hydrogel granule 24, as obtained in the above way, preferably has anaverage particle diameter of 0.3 to 10 mm, more preferably, 0.5 to 8 mm,particularly preferably, 1 to 5 mm. In the case where the averageparticle diameter of the hydrogel granule 24 is smaller than 0.3 mm, thegranulation ratio might be low, and further the granulation strength ofa dry granule (water-absorbent resin granule), as obtained by drying thehydrogel granule 24, might be insufficient. In addition, in the casewhere the average particle diameter of the hydrogel granule 24 is largerthan 10 mm, the physical properties might be deteriorated, or the finepowder content might be large.

That is to say, for the purpose of obtaining a water-absorbing agenthaving still more excellent granulation strength and excellent physicalproperties such as the absorption capacity under a load or theabsorption speed, it is preferable to obtain a particulate hydrogelgranule 24 with a moderate particle diameter and then shrink theresultant hydrogel granule 24 by drying.

As is aforementioned, thus, the hydrogel granule 24 as obtained in theabove way is dried, classified, and then, as is or aftersurface-crosslinked, finished up to a product as a water-absorbent resinwith excellent strength properties. If the hydrogel granule 24 is dried,its granulation strength can be enhanced, whereby the fine powder isunited more strongly and thereby regenerated with as high a strength asa primary particle.

Incidentally, in the present invention, the hydrogel granule is such ashas a water content of at least 10% by weight of the entire hydrogelgranule. The hydrogel granule may be formed into a water-absorbent resingranule (dry granule) with a water content less than 10% by weight bydrying.

The method for the above drying is not specifically limited, andconventional dryers or ovens are widely used. The drying temperature is,preferably, relatively high, concretely, in the range of 110 to 300° C.,more preferably, 120 to 200° C., particularly preferably, 150 to 180° C.If the hydrogel granule 24 is dried, its shrinkage occurs, and as aresult, a strong water-absorbent resin granule can be obtained.

The period of time to dry the hydrogel granule 24 is preferably notshorter than a certain period of time, more preferably, in the range of5 minutes to 10 hours, in view of physical properties. In addition,after drying, the solid content is preferably not less than 90% byweight. Incidentally, the dry-treatment may be carried out either foronly the hydrogel granule 24, as produced by the continuous granulationprocess of the present invention, or for a combination thereof with thehydrogel polymer which is obtained by the above-mentioned aqueoussolution polymerization or reversed-phase suspension polymerization andhas not yet been dried.

Incidentally, in the present invention, the water-absorbent resingranule is a particulate aggregate with a specific particle size asformed by aggregation of a plurality of water-absorbent resin powders 21with the aqueous liquid 22. Incidentally, that the resultant mixture ofthe water-absorbent resin powder 21 and the aqueous liquid 22 is agranule (fine powder aggregate) can be judged from a fact that theaggregation of individual particles (of water-absorbent resin powder 21)can be confirmed with a optical micrograph of the hydrogel granule 24 oran electron micrograph as taken without pulverizing a dried product ofthe hydrogel granule 24, or from a fact that the particles swell as aplurality of discontinuous particles in a large excess of water or anaqueous liquid.

Incidentally, the above embodiment of the present invention has aconstitution such that the discharge outlet 10 of the continuousextrusion mixer 1 is formed in the bottom wall of the casing 2, but theformation position of the discharge outlet 10 is not necessarily limitedto in the bottom wall of the casing 2, but may, for example, be in anend face toward which the water-absorbent resin powder is conveyed inthe casing 2.

Furthermore, the shape of the casing 2, namely, the shape of thecontinuous extrusion mixer, or the direction in which the casing 2 isfixed (set), is not especially limited, either, and may, for example, bethe shape such that the casing 2 is fixed vertically (namely, inparallel with the gravity direction). In this case, the gravity is addedto the extrusion thrust, whereby it might be possible to discharge theresultant hydrogel granule 24 more smoothly.

As is mentioned above, in the continuous granulation process of thepresent invention, when the water-absorbent resin powder and the aqueousliquid are supplied into the continuous extrusion mixer having aplurality of supplying-inlets along an arrangement of stirring-membersand mixed therein, the water-absorbent resin powder is supplieddownstream of the aqueous liquid, whereby the amount of the adhesivematerials adhering to the continuous extrusion mixer can be reduced tocarry out stable granulation continuously for a long term, and further agranule with excellent granulation strength can be obtained.Incidentally, in the case where the water-absorbent resin powder and theaqueous liquid are not sufficiently uniformly mixed, the aqueous liquidis not sufficiently absorbed into the central portion of thewater-absorbent resin powder, and therefore the resultant granulationstrength is insufficient, so the amount reverting to a ungranulatedstate increases due to drying and pulverizing steps. However, in thecontinuous granulation process of the present invention, a granule withas high a strength as a primary particle can be obtained.

(Effects and Advantages of the Invention)

The present invention displays the following excellent properties (1) to(7).

(1) Improvement of surface-crosslinking effects, prevention ofreproduction of fine powder, and decrease of fine powder content: In thecase where the granulation is carried out by conventional processes,such as a process comprising granulation after crosslinking the surfaceneighborhood of a water-absorbent resin and a process comprising thesimultaneous steps of the granulation and the surface-crosslinking ofthe water-absorbent resin, the resultant granule is fractured due tomechanical stress during or after the granulation, resulting in thefracture of the surface crosslinkage and in the deterioration of thephysical properties. In contrast therewith, in the present invention, afine powder portion is first separated from the water-absorbent resinand then granulated, and the resultant granule is mixed with a primaryparticle of the water-absorbent resin residue as obtained by removingthe fine powder from the water-absorbent resin in the above way, and theresultant mixture is treated with the surface-crosslinking agent.Therefore, the surface-crosslinking agent is uniformly distributed overthe entire particles, so the water-absorbent resin composition withexcellent physical properties can be obtained. In other words, becausethe granulation strength of the granule is high, the granulationfracture due to mechanical stress is difficult to occur, and as aresult, the surface crosslinkage is difficult to fracture, and furtherthe resultant water-absorbent resin composition reproduces the finepowder only a little and has only a low fine-powder content.

(2) Enhancement of physical strength of composition: Probably becausethe primary particle with high particle strength against mechanicalstress supports the entire composition, the granulation fracture due tothe mechanical stress of the water-absorbent resin granule in thecomposition is difficult to occur. Therefore, the resultantwater-absorbent resin composition displays enhanced physical strengthwhen compared with those which are obtained by surface-crosslinkingeither the primary particle containing the fine powder or the granulealone.

(3) Synergistic effects of physical properties: Conventionalwater-absorbent resins contain fine powders and therefore has alimitation in the improvement of physical properties. The removal of thefine powder is not only economically disadvantageous, but also reducesthe surface area of the residue (water-absorbent resin primary particle)and therefore lowers the water absorption speed thereof. The productionof only the water-absorbent resin granule comprises complicated processsteps and involves a high cost of energy. However, if thewater-absorbent resin primary particle and the water-absorbent resingranule, which is a granulation product of the fine powder, are obtainedfrom the the water-absorbent resin and then mixed and thensurface-crosslinked in accordance with the present invention, thewater-absorbent resin composition with a high absorption capacity undera load as well as a fast absorption speed can be obtained.

(4) Enhancement of granulation strength: The enhancement in thegranulation strength or in the absorption capacity under a load canfurther be designed using specific granulation processes.

(5) Excellent physical properties: Granules as obtained by conventionalgranulation processes can bear only a low load of at most about 20 g/cm²because of the granulation fracture, but the water-absorbent resingranule as obtained in the present invention displays excellentabsorption even under a high load of 50 g/cm². Therefore, because ofcontaining such a granule, the water-absorbent resin composition of thepresent invention displays a higher absorption capacity under a highload of 50 g/cm² than conventional ones, as well as excellent absorptionspeed, and further is free from the fine powder. In addition, thewater-absorbent resin composition of the present invention preferablyhas the following properties: an absorption speed of 100 seconds orless; a water-soluble content of 15% by weight or lower, morepreferably, 10% by weight or lower; a particle size distribution of 95%by weight or higher, more preferably, 98% by weight or higher, in termsof the proportion of particles with a particle size of 850 to 150 μm;and a granulation fracture ratio of 10% or less. Incidentally, themeasurement methods for these physical properties are specified in thebelow-mentioned examples of some preferred embodiments according to thepresent invention.

(6) The continuous granulation process of the present invention makeseffects to reduce the amount of the adhesive materials, adhering to thecontinuous extrusion mixer, to thereby carry out stable granulationcontinuously for a long term, and further to obtain a granule withexcellent granulation strength.

(7) WO96/13542 discloses a process in which a water-absorbent resinpowder gets, first, surface-crosslinked, and then classified, thusobtaining a primary particle (which has been surface-crosslinked) of alarge particle diameter on a sieve, while granulating a fine particle(which has been surface-crosslinked) as passed through the sieve, andthen the resultant granule (which has been surface-crosslinked) is mixedwith the above primary particle which has been surface-crosslinked. Asis mentioned above, in such a conventional process, a water-absorbentresin powder, first, gets surface-crosslinked. Because thesurface-crosslinked water-absorbent resin powder has not yet beenclassified in this stage, however, it contains a particle of a verysmall particle diameter. Specifically describing, thesurface-crosslinked water-absorbent resin powder in that stage containsnot only a particle of a large particle diameter (i.e. particle obtainedas a primary particle), but also a particle of a very small particlediameter (i.e. fine particle). Therefore, in the case where an aqueousliquid containing a surface-crosslinking agent is added to such awater-absorbent resin powder, the fine particle absorbs the aqueousliquid more than the primary particle, and therefore is stronglycrosslinked. Thus, as to the particle of a large particle diameter, onlyits surface is crosslinked, whereas the fine particle is crosslinked upto its inside. The foregoing granule comprises such a fine particle ascrosslinked up to its inside, and therefore is a particle inferior withregard to the water absorbency because of such an excessivecrosslinking. In comparison therewith, as to the present invention, thegranulation is beforehand carried out to enlarge the particle diameter,and then the surface-crosslinking is carried out, so the crosslinking ofthe granule can be prevented from advancing up to the inside of thegranule.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention is more specifically illustrated bythe following examples of some preferred embodiments in comparison withcomparative examples not according to the invention. However, theinvention is not limited to the below-mentioned examples.

The physical properties of water-absorbent resins were measured asfollows:

(Water absorption capacity)

A nonwoven fabric bag (60 mm×60 mm), in which about 0.2 g of awater-absorbent resin powder or composition was put uniformly, wasimmersed into a 0.9 wt % aqueous sodium chloride solution (physiologicalsalt solution). After 60 minutes, the bag was drawn up and then drainedat 250 G with a centrifuge for 3 minutes. Then, weight W₁ (g) of the bagwas measured. In addition, the same procedure as the above was carriedout using no water-absorbent resin, and weight W₀ (g) of the resultantbag was measured.

Thus, the water absorption capacity (g/g) was calculated from theseweights W₁ and W₀ in accordance with the following equation:

water absorption capacity (g/g)=(weight W₁ (g)−weight W₀ (g))/0.2 (g).

(Water-soluble content)

First, 0.50 g of a water-absorbent resin powder or composition wasdispersed into 1,000 ml of deionized water, and stirred for 16 hours,and then filtered with a filter paper (TOYO, No.6). The solid content inthe resultant filtrate was measured to calculate the water-solublecontent in accordance with the following equation:

water-soluble component (wt %)=(liquid weight (g))×(solid content infiltrate (wt %))/0.5 (g).

(Absorption capacity under load)

Hereinafter, a measurement apparatus as used for measuring theabsorption capacity under a load is explained while referring to FIG. 1.

As is shown in FIG. 1, the measurement apparatus comprises: a scale 21;a vessel 22 of a predetermined capacity as mounted on the scale 21; anair-inhaling pipe 23; an introducing tube 24; a glass filter 26; and ameasurement part 25 as mounted on the glass filter 26.

The vessel 22 has an opening part 22 a on the top and an opening part 22b on the side. The air-inhaling pipe 23 is inserted in the opening part22 a of the vessel 22, and the introducing tube 24 is fitted to theopening part 22 b.

In addition, the vessel 22 contains a predetermined amount ofphysiological salt solution 32. The lower part of the air-inhaling pipe23 is submerged in the physiological salt solution 32. The air-inhalingpipe 23 is fitted to keep the internal pressure of the vessel 22 almostatmospheric. The glass filter 26 is formed in a diameter of 55 mm. Thevessel 22 and the glass filter 26 are connected to each other throughthe introducing tube 24 made of a silicone resin. In addition, theposition and the level of the glass filter 26 are fixed relative to thevessel 22.

The measurement part 25 comprises: a filter paper 27; a supportingcylinder 28; a wire net 29 as attached to the bottom of the supportingcylinder 28; and a weight 30; and the measurement part 25 is formed bymounting the filter paper 27 and the supporting cylinder 28, as bottomedwith a wire net 29, in this order on the glass filter 26 and furthermounting the weight 30 inside the supporting cylinder 28, namely, on thewire net 29. The wire net 29 is made of stainless steel and formed in400 mesh (mesh size: 38 μm). In addition, the upper face, namely, thecontact face between the wire net 29 and a water-absorbent resincomposition 31, of the wire net 29 is set to be as high as a lower part23 a of the air-inhaling pipe 23. An arrangement is made such that apredetermined amount of water-absorbent resin composition with apredetermined particle diameter can uniformly be spread on the wire net29. The weight 30 is adjusted in weight such that a load of 50 g/cm² canuniformly be applied to the water-absorbent resin composition 31 on thewire net 29.

The absorption capacity under a load was measured with the measurementapparatus having the above-mentioned constitution. The measurementmethod is hereinafter explained.

First, predetermined preparatory operations are made, in which, forexample, a predetermined amount of the physiological salt solution 32 isplaced into the vessel 22, and the air-inhaling pipe 23 is inserted intothe vessel 22. Next, the filter paper 27 is mounted on the glass filter26, and further, in parallel with this mounting operation, 0.9 g ofwater-absorbent resin composition is uniformly spread inside thesupporting cylinder 28, namely, on the wire net 29, and the weight 30 isthen put on the water-absorbent resin composition 31.

Next, the wire net 29 of the supporting cylinder 28, on which thewater-absorbent resin composition 31 and the weight 30 are put, ismounted on the filter paper 27 concentrically with the glass filter 26.

Then, the weight of the physiological salt solution 32, as absorbed bythe water-absorbent resin composition 31 over a period of 60 minutessince the supporting cylinder 28 is mounted on the filter paper 27, isdetermined from a measured value with the scale 21. The same procedureas the above was carried out without using the water-absorbent resincomposition 31, and the weight of the physiological salt solution 32, asabsorbed by materials other than the water-absorbent resin composition31, such as the filter paper 27, was determined from a measured valuewith the scale 21 and regarded as a blank value. Subsequently, thecorrection by subtracting the blank value was carried out, and the netweight of the physiological salt solution 32, as absorbed by thewater-absorbent resin composition 31, was divided by the weight of thewater-absorbent resin composition 31 (0.9 g), thus calculating anabsorption capacity (g/g) under a load of 50 g/cm².

(Water absorption speed)

First, various reagents were dissolved into water, thus preparing anaqueous solution containing sodium cation of 600 to 700 ppm, calciumcation of 65 to 75 ppm, magnesium cation of 55 to 65 ppm, potassiumcation of 1,100 to 1,200 ppm, phosphorus of 240 to 280 ppm, sulfur of450 to 500 ppm, chlorine of 1,100 to 1,300 ppm, and sulfuric acid ion of1,300 to 1,400 ppm. This aqueous solution was used as an artificialurine.

Next, 0.358 g of a water-absorbent resin composition, as classified into300 to 850 μm with a JIS standard sieve, was placed into a glass-madetest tube (inner diameter=about 14.1 mm, height=about 126 mm), and 10.00g of the above artificial urine was then poured into the test tube atonce. The number of the seconds in time, which had passed until theentirety of the 10.00 g of the artificial urine had been absorbed by the0.358 g of the water-absorbent resin composition and had formed aswollen gel of 28 times the original, was measured and regarded as theabsorption speed.

(Granulation strength)

The granulation strength was measured by a method as disclosed inJP-A-09-235378. Hereinafter, the way to apply the impact force (B), asdisclosed therein, is specifically explained.

A vessel, as used when applying the impact force (B) to the abovewater-absorbent resin granule, has an inner cap and an outer cap on atransparent glass-made vessel body with a height of about 10.8 cm, adiameter of about 6.2 cm, and a capacity of 225 g. As to such a vessel,for example, a so-called mayonnaise bottle (trade name: A-29), made byYamamura Glass K.K., is favorably used. In addition, preferable examplesof the above glass beads are those which are made of soda-lime glass andused as fractional distillation fillers with an average bead diameter ofabout 6 mm as made uniform within the bead diameter range of about 5.9to about 6.4 mm. Incidentally, 10.0 g of the above glass beadscorrespond to 31 to 33 in number of the glass beads.

When the impact force (B) is applied to a water-absorbent resincomposition, 30.0 g thereof is placed into the vessel body of the abovevessel along with 10.0 g of the above glass beads, and then the innerand outer caps are both closed. Then, this vessel is fixed to adispersing machine (No. 488 dispersing machine for test, made by ToyoSeiki Seisakusho K.K.) by interposing the vessel between an upper and alower clamp as equipped to the dispersing machine, and a vibration of avibration speed revolution number of 750 c.p.m. is given to the vesselusing an alternating electric power source of 100 V/60 Hz for 30minutes. As a result, the vessel, as fixed to the dispersing machine,pivots at angles of 12.5° left and right each (25° in total) to a faceto which the upper and the lower clamp are fitted, and simultaneouslytherewith, the vessel vibrates 8 mm back and forth each (16 mm intotal), thus applying the impact force to the water-absorbent resincomposition in the vessel.

The fracture ratio of the water-absorbent resin composition (hereinafterreferred to as “granulation fracture ratio”) is a percentage value asdetermined by measuring the weight of a portion, as fractured due to theabove application of the impact force (B) for 30 minutes and the abovevibration along with the glass beads, of the water-absorbent resincomposition in the vessel and by dividing the above-measured weight ofthe fractured portion of the vibrated water-absorbent resin compositionby the original weight of the water-absorbent resin composition ascharged.

Thus, the above granulation fracture ratio can be determined bymeasuring, by ROTAP Sieve Tester with JIS standard sieves, the weight ofa particle with a certain particle size (e.g., what passed throughmeshes of 150 μm) resultant from the fracture by applying the impactforce (B).

Production Example 1-1

For Water-Absorbent Resin Powder

An aqueous solution was prepared by dissolving polyethylene glycoldiacrylate of 0.05 mol % as an internal crosslinking agent into 5,500 gof an aqueous solution of sodium acrylate with a neutralization ratio of75 mol % (monomer concentration: 33 wt %), and then degassed with anitrogen gas for 30 minutes, and then supplied into a reaction vessel asprepared by capping a stainless-steel-made double-arm type kneader of acapacity of 10 liters having two sigma type wings and a jacket. Whilemaintaining the reaction system at 20° C., the replacement with anitrogen gas in the reaction system was continued. Next, while rotatingthe wings, 2.9 g of sodium persulfate and 0.16 g of L-ascorbic acid wereadded in the form of 10 wt % aqueous solutions of them each. As aresult, 1 minute after, a polymerization reaction got started and, 16minutes after, the reaction system reached the peak temperature of 83°C., when the resultant hydrogel polymer was a finely particulated onewith a size of about 5 mm. Then, the stirring was further continued, andthe resultant hydrogel polymer was separated out 60 minutes after theinitiation of the polymerization.

The resultant finely-particulated hydrogel polymer was spread on a wirenet with a mesh size of 300 μm (50 mesh) and then dried at 150° C. withhot air for 90 minutes. Then, the resultant dried product was pulverizedwith a roller mill, and then classified with a mesh of 850 μm, thusobtaining a pulverized water-absorbent resin powder (A) with an averageparticle diameter of 300 μm, a particle diameter distribution where theproportion of the resin with a particle diameter smaller than 150 μm was15% by weight, and a water content of 6% by weight. Then, thewater-absorbent resin powder (A) was classified with a sieve of the meshsize of 150 μm into a water-absorbent resin powder (A₁) of 850 to 150 μmand a water-absorbent resin powder (A₂) smaller than 150 μm.Incidentally, the powders (A₁) and (A₂) are a primary particle and afine powder, respectively, as referred to in the present invention. Thewater-absorbent resin powder (A) showed a water absorption capacity of42 g/g and a water-soluble content of 10% by weight.

Production Example 1-2

For Water-absorbent Resin Powder

A reaction solution was prepared by dissolving trimethylolpropanetriacrylate of 0.04 mol % as an internal crosslinking agent into 5,500 gof a 38 wt % aqueous solution of sodium acrylate (neutralization ratio:75 mol %) as a monomer component in the same polymerization vessel asused in Production Example 1-1. Next, 2.9 g of ammonium persulfate and0.02 g of L-ascorbic acid were added to the above reaction solution tocarry out a polymerization reaction in the same way as of ProductionExample 1-1. The resultant hydrogel polymer was dried in the same way asof Production Example 1-1 and then pulverized with a roll granulatortype pulverizer as equipped with three pulverizing rolls which werestepwise arranged at predetermined intervals (roll gaps: about 1.63 mm,about 0.43 mm, and about 0.15 mm). Then, the resultant pulverizationproduct was classified with a JIS standard sieve of a mesh size of 850μm, thus obtaining a pulverized water-absorbent resin powder (B) with anaverage particle diameter of 300 μm. This water-absorbent resin powder(B) showed a water absorption capacity of 33 g/g and a water-solublecontent of 10 wt %.

The water-absorbent resin powder (B) was further classified with a JISstandard sieve of a mesh size of 150 μm, thus obtaining awater-absorbent resin powder (B₁) of 86.3 wt % with a particle diameterof 850 to 150 μm and a water-absorbent resin powder (B₂) of 13.7 wt %with a particle diameter smaller than 150 μm.

Granulation Examples and Compositions Using Them Granulation Example 1

Granulation with Preheated Aqueous Liquid

First, 200 g of the water-absorbent resin powder (A₂) with a particlediameter smaller than 150 μm, as obtained in Production Example 1-1above, was placed into a mortar mixer of 5 liters (the temperature of avessel of 5 liters in the mixer was kept with a bath of 70° C.) asproduced by Nishi Nihon Shikenki Seisakusho K.K. Then, 300 g of water,as heated to 90° C., was added at once while rotating the impeller ofthe above mortar mixer at a high speed using an alternating electricpower source of 60 Hz/100 V.

The water-absorbent resin powder (A₂) was mixed with water within 10seconds, and the resultant entire contents of the mixer was a gelatinouswater-absorbent resin granule with a particle diameter of about 3 toabout 10 mm. In the mortar mixer, the water-absorbent resin granule wasin pieces, and there was no sign that the granule was kneaded by mixingwith the impeller.

After the high-speed stirring in the mortar mixer for 3 minutes, theresultant water-absorbent resin granule in pieces was separated from themixer, and then placed on a wire net of the mesh size of 300 μm, andthen dried in a hot-air circulation type dryer at 150° C. for 2 hours.Next, the resultant dry granule was pulverized with a roller mill underthe same conditions as those in Production Example 1-1, and thenclassified into particles with a particle diameter of 850 to 150 μm,thus obtaining a water-absorbent resin granule (1) with a waterabsorption capacity of 42 g/g and a water-soluble content of 10% byweight. The ratio of the water-absorbent resin granule with a particlediameter of 850 to 150 μm was 83% of the roller mill pulverizationproduct. Incidentally, a few particles of the water-absorbent resingranule (1) were extracted, and the physiological salt solution was thenadded dropwise to each particle, and the liquid-absorption behavior wasthen observed. As a result, the collapse to small fine particles wasseen with the progress of swelling.

Granulation Example 2

Granulation with Unheated Water

The same procedure as of Granulation Example 1 was carried out exceptthat the water to be added to 200 g of the water-absorbent resin powder(A₂) was gradually added with a spray, in other words, except that ittook 30 minutes to add 300 g of water.

As water was added, the water-absorbent resin powder (A₂) grewaggregated to form a huge mass with a size of 20 to 50 mm, and finally,entirely became lumped and got kneaded. The resultant lumpy hydrogelaggregate was separated from the mixer, and then sliced into the size ofnot larger than 10 mm with a cutter, and then placed on a wire net ofthe mesh size of 300 μm, and then dried in a hot-air circulation typedryer at 150° C. for 2 hours.

Then, the resultant dry product was pulverized with a roller mill, andthen classified into particles with a particle diameter of 850 to 150μm, thus obtaining a water-absorbent resin granule (2) with a waterabsorption capacity of 42 g/g and a water-soluble content of 14% byweight. A few particles of the water-absorbent resin granule (2) wereextracted, and the physiological salt solution was added dropwise toeach particle, and the absorption behavior was observed. The particlesswelled slowly without collapse.

Comparative Example 1-1

Surface-crosslinking of Granule Alone

An aqueous liquid of surface-crosslinking agents, comprising 0.05 partsby weight of ethylene glycol diglycidyl ether, 1.0 part by weight ofpropylene glycol, 3 parts by weight of water, and 0.9 parts by weight ofisopropanol, was mixed with 100 parts by weight of the water-absorbentresin granule (1) as obtained in Granulation Example 1. The resultantmixture was heated at 195° C. for 30 minutes, thus obtaining acomparative water-absorbent resin composition (1) with a waterabsorption capacity of 33 g/g and an absorption capacity of 27 g/g undera load. However, when this swollen gel was strongly pushed with afinger, the granulation was fractured, and a fine gel with poorliquid-permeability was formed.

Comparative Example 1-2

Surface-crosslinking of Primary Particle Alone

The same surface-crosslinking as of Comparative Example 1-1 was carriedout to 100 parts by weight of the water-absorbent resin powder (A₁)which was a primary particle as obtained in Production Example 1-1. As aresult, a comparative water-absorbent resin composition (2) with a waterabsorption capacity of 33 g/g and an absorption capacity of 27 g/g undera load was obtained.

Comparative Example 1-3

Surface-crosslinking of Mixture of Primary Particle and Fine Powder

The same procedure as of Comparative Example 1-1 was carried out to thewater-absorbent resin powder (A) with a particle diameter smaller than850 μm as obtained in Production Example 1-1 above. As a result, acomparative water-absorbent resin composition (3) with a waterabsorption capacity of 33 g/g and an absorption capacity of 24 g/g undera load was obtained.

EXAMPLE 1-1

Surface-crosslinking of Particle Mixture

The same procedure as of Comparative Example 1-1 was carried out to 100parts by weight of a particle mixture comprising 15 parts by weight ofthe water-absorbent resin granule (1) as obtained in Granulation Example1 and 85 parts by weight of the water-absorbent resin powder (A₁) whichwas a primary particle as obtained in Production Example 1-1. As aresult, a water-absorbent resin composition (1) was obtained.

Granulation Example 3

Granulation with Specific Mixer

The water-absorbent resin fine powder (B₂), as obtained in ProductionExample 1-2 above, and ion-exchanged water were continuously mixed byprojecting the water-absorbent resin fine powder (B₂) into thecontinuous extrusion mixer 1 of FIG. 2 at a rate of 2 kg/minute and, inparallel therewith, projecting the ion-exchanged water from theliquid-supplying inlet 4 with a diameter of 5 mm of the above continuousextrusion mixer 1 in a ratio of 130 parts by weight per 100 parts byweight of the water-absorbent resin fine powder (B₂). As a result, aparticulate uniform gelatinous water-absorbent resin granule wascontinuously discharged from the discharge outlet. The resultantparticulate gelatinous granule was an aggregate of each particle, andmost thereof was a uniform gelatinous granule with a particle diameterof about 1 mm to about 5 mm. In addition, the above gelatinous granulehad a solid content of 43.6% by weight. Incidentally, the solid contentof the gelatinous granule is the amount (content) of the water-absorbentresin in the gelatinous granule.

The above gelatinous granule was spread into a thickness of about 5 cmon a JIS standard wire net of the mesh size of 300 μm and then driedwith a hot-air circulation type dryer of 160° C. As a result, the abovegelatinous granule was dried uniformly and sufficiently so as to have asolid content of at least 90 wt %, thus obtaining a powdery dry granuleof which the particles could easily be pulverized even by hand. Theproportion of lumps larger than 10 mm in the dry granule was only 5%.

Next, this dry granule was pulverized with the foregoing roller mill(however, the roll gaps were widened and finally evened to about 0.27mm) and then classified with a JIS standard sieve of a mesh size of 850μm, thus obtaining a water-absorbent resin granule (3).

The particle size distribution was measured for the water-absorbentresin granule (3), the water-absorbent resin powder (B), thewater-absorbent resin powder (B₁), and the water-absorbent resin finepowder (B₂), as obtained in the above ways, and results thereof areshown in Table 2. In addition, in spite of the use of thewater-absorbent resin fine powder (B₂), the water-absorbent resingranule (3) was a granule (aggregate) of particles, of which about 80%had a particle diameter of 300 to 850 μm, and as a result, thewater-absorbent resin granule (3) was a granule (aggregate) which hadsuch a high granulation strength that the granulation fracture ratio, asdefined by the impact force (B), was 2.4 wt %.

Granulation Example 4

Granulation with Preheated Aqueous Liquid in Specific Mixer

The same procedure as of Granulation Example 3 was carried out exceptthat the temperature of the ion-exchanged water, as used for thegranulation, was changed from room temperature to 90° C. As a result, anadhesion of the water-absorbent resin to the mixer, which had slightlybeen seen in Granulation Example 3, was hardly seen in the presentgranulation example, so the continuous granulation ability was furtherimproved.

Comparative Example 1-4

Surface-crosslinking of Water-absorbent Resin Granule Alone

A surface-crosslinking agent, comprising 0.05 parts by weight ofethylene glycol diglycidyl ether, 0.75 parts by weight of glycerol, 3parts by weight of water, 0.75 parts by weight of isopropanol, and 0.5parts by weight of lactic acid, was mixed with 100 parts by weight ofthe water-absorbent resin granule (3) as obtained in Granulation Example3. The resultant mixture was heated at 200° C. for 40 minutes, thusobtaining a comparative water-absorbent resin composition (4) with awater absorption capacity of 28 g/g and an absorption capacity of 23 g/gunder a load.

Comparative Example 1-5

Surface-crosslinking of Primary Particle Alone

A comparative water-absorbent resin composition (5) was obtained bycarrying out the same surface-crosslinking as of Comparative Example 1-4to 100 parts by weight of the water-absorbent resin powder (B₁) whichwas a primary particle as obtained in Production Example 1-2. Theresultant comparative water-absorbent resin composition (5) showed awater absorption capacity of 28 g/g and an absorption capacity of 25 g/gunder a load.

Comparative Example 1-6

Surface-crosslinking of Mixture of Primary Particle and Fine Powder

A comparative water-absorbent resin composition (6) was obtained bycarrying out the same surface-crosslinking as of Comparative Example 1-4to the water-absorbent resin powder (B) with a particle diameter smallerthan 850 μm as obtained in Production Example 1-2 above. The resultantcomparative water-absorbent resin composition (6) showed a waterabsorption capacity of 28 g/g and an absorption capacity of 22 g/g undera load.

EXAMPLE 1-2

Surface-crosslinking of Particle Mixture

A water-absorbent resin composition (2) was obtained by carrying out thesame procedure as of Comparative Example 1-2 to 100 parts by weight of aparticle mixture comprising 13.7 parts by weight of the water-absorbentresin granule (3) as obtained in Granulation Example 3 and 86.3 parts byweight of the water-absorbent resin powder (B₁) which was a primaryparticle as obtained in Production Example 1-2. The resultantwater-absorbent resin composition (2) showed a water absorption capacityof 28 g/g and an absorption capacity of 25 g/g under a load.

EXAMPLE 1-3

Surface-crosslinking of Particle Mixture

The water-absorbent resin powder (1), as obtained in Example 2-1 below,was classified with a sieve of the mesh size of 500 μm and a sieve ofthe mesh size of 150 μm, thus obtaining a water-absorbent resin granulewith a particle diameter of 500 to 150 μm. Then, a water-absorbent resincomposition (3) was obtained by carrying out the same procedure as ofComparative Example 1-1 to 100 parts by weight of a particle mixturecomprising 15 parts by weight of the water-absorbent resin granule asobtained immediately above and 85 parts by weight of the water-absorbentresin powder (A₁) which was a primary particle as obtained in ProductionExample 1-1.

EXAMPLE 1-4

Mixing of Surface-crosslinked Product of Water-absorbent Resin PrimaryParticle and Surface-crosslinked Product of Water-absorbent ResinGranule

A water-absorbent resin composition (4) was obtained by uniformly mixing40 parts by weight of the comparative water-absorbent resin composition(1), as obtained in Comparative Example 1-1, with 60 parts by weight ofthe comparative water-absorbent resin composition (5) as obtained inComparative Example 1-5.

Results of the Examples and the Comparative Examples, as mentionedabove, are shown in Table 1.

As is shown in Table 1, the water-absorbent resin compositions,according to the present invention, show an excellent absorption of atleast 25 g/g even under a high load of 50 g/cm² unlike conventional onesnot according to the present invention, and further are very excellentin the absorption speed and the particle size distribution because ofthe granule content. In addition, as to the examples in which theparticle mixtures were used according to the present invention, becausethe primary particle with high mechanical strength supports the entirecomposition, the fracture of the swollen gel that is seen in ComparativeExamples 1-1 and 1-4 (cases of granule alone) is substantially not seen.

TABLE 1 Water Absorption Water Granu- absorption capacity absorptionlation capacity under load speed Gel fracture (g/g) (g/g) (seconds)fracture ratio (%) Comparative 33 27 50 seen 2.5 Example 1-1 (granulealone) Comparative 33 27 140 none 0 Example 1-2 (powder (A₁) alone)Comparative 33 24 120 none — Example 1-3 (powder (A)) Example 1-1 33 2795 none 0.3 (particle mixture) Comparative 28 23 50 seen 2.1 Example 1-4(granule alone) Comparative 28 25 140 none 0 Example 1-5 (powder (B₁)alone) Comparative 28 22 120 none — Example 1-6 (powder (B)) Example 1-228 25 90 none 0.2 (particle mixture) Example 1-3 33 27 85 none 0.1(particle mixture) Example 1-4 31 26 95 partially 0.9 (particle seenmixture)

In addition, the respective particle size distribution of the powder(B), the powder (B₁), the fine powder (B₂), and the granule (3), asobtained above, are shown in Table 2 below.

TABLE 2 Particle size 850/ 500/ 300/ 150/ distribution (μm) 500 300 15075 75 or less Powder (B) 7.0 42.3 37.0 9.8 3.9 Powder (B₁) 8.1 49.1 42.90 0 Fine powder 0 0 0 71.5 28.5 (B₂) Granule (3) 19.7 58.0 15.6 5.2 2.4

Granulation Example 5

First, 400 g of the water-absorbent resin powder (A₂) with a particlediameter smaller than 150 μm, as obtained in Production Example 1-1, wasplaced into Lödige Mixer M-5 (trademark) of 5 liters as produced byGebrüder Lödige Maschinenbau GmbH. Then, 600 g of water, as heated to90° C., was injected from a funnel at once while rotating the impellerof the above mixer at a high speed using an alternating electric powersource of 60 Hz/100 V.

The water-absorbent resin powder (A₂) was mixed with water within 10seconds, and the resultant entire contents of the mixer was a gelatinouswater-absorbent resin granule with a particle diameter of about 1 toabout 5 mm. In the Löbdige Mixer, the water-absorbent resin granule wasin pieces, and there was no sign that the granule was kneaded by mixingwith the impeller.

After the high-speed stirring in the Lödige Mixer for 1 minute, theresultant water-absorbent resin granule was separated from the mixer,and then dried in the same way as of Granulation Example 1. As a result,the granule was uniformly and sufficiently dried.

Next, the resultant dry granule was pulverized with the foregoing rollermill under the same conditions as those in Production Example 1-1, andthen classified into particles with a particle diameter of 850 to 150μm, thus obtaining a water-absorbent resin granule (5) with a waterabsorption capacity of 42 g/g and a water-soluble content of 10% byweight. The ratio of the water-absorbent resin granule with a particlediameter of 850 to 150 μm was 82% of the pulverization product.

A few particles of the water-absorbent resin granule (5) were extracted,and the physiological salt solution was then added dropwise to eachparticle, and the liquid-absorption behavior was then observed. As aresult, the collapse to small fine particles was seen with the progressof swelling.

Granulation Example 6

A water-absorbent resin granule in pieces in the same state as ofGranulation Example 1 was obtained in the same way as of GranulationExample 1 except that the amount of the heated water was 200 g. Theresultant water-absorbent resin granule was treated in the same way asof Granulation Example 1 and then classified into particles with aparticle diameter of 850 to 150 μm, thus obtaining a water-absorbentresin granule (6) with a water absorption capacity of 42 g/g and awater-soluble content of 11% by weight. The ratio of the water-absorbentresin granule (6) with a particle diameter of 850 to 150 μm was 80% ofthe pulverization product from the roller mill.

Granulation Example 7

A water-absorbent resin granule in pieces in the same state as ofGranulation Example 1 was obtained in the same way as of GranulationExample 1 except that the amount of the heated water was 450 g. Theresultant water-absorbent resin granule was treated in the same way asof Granulation Example 1 and then classified into particles with aparticle diameter of 850 to 150 μm, thus obtaining a water-absorbentresin granule (7) with a water absorption capacity of 42 g/g and awater-soluble content of 10% by weight. The ratio of the water-absorbentresin granule (7) with a particle diameter of 850 to 150 μwas 84% of thepulverization product from the roller mill.

Granulation Example 8

A water-absorbent resin granule in pieces in the same state as ofGranulation Example 1 was obtained in the same way as of GranulationExample 1 except that the temperature of the heated water was 70° C. Theresultant water-absorbent resin granule was treated in the same way asof Granulation Example 1 and then classified into particles with aparticle diameter of 850 to 150 μm, thus obtaining a water-absorbentresin granule (8) with a water absorption capacity of 42 g/g and awater-soluble content of 10% by weight. The ratio of the water-absorbentresin granule (8) with a particle diameter of 850 to 150 μm was 84% ofthe pulverization product from the roller mill.

Granulation Example 9

First, 300 g of water of 80° C. was placed into a mortar mixer of 5liters (the temperature of a vessel of 5 liters in the mixer was keptwith a bath of 80° C.) as produced by Nishi Nihon Shikenki SeisakushoK.K. Then, 200 g of the water-absorbent resin powder (A₂) with aparticle diameter smaller than 150 μm, as obtained in Production Example1-1, was added at once while rotating the impeller of the above mortarmixer at a high speed using an alternating electric power source of 60Hz/100 V.

The water-absorbent resin powder (A₂) was mixed with water within 10seconds, and the resultant entire contents of the mixer was a gelatinouswater-absorbent resin granule with a particle diameter of about 3 toabout 10 mm. In the mortar mixer, the water-absorbent resin granule wasin pieces, and there was no sign that the granule was kneaded by mixingwith the impeller.

After the high-speed stirring in the mortar mixer for 3 minutes, theresultant water-absorbent resin granule in pieces was separated from themixer, and then placed on a wire net of the mesh size of 300 μm (50mesh), and then dried in a hot-air circulation type dryer at 150° C. for2 hours.

Next, the resultant dry granule was pulverized with the foregoing rollermill under the same conditions as those in Production Example 1-1, andthen classified into particles with a particle diameter of 850 to 150μm, thus obtaining a water-absorbent resin granule (9) with a waterabsorption capacity of 42 g/g and a water-soluble content of 10% byweight. The ratio of the water-absorbent resin granule (9) with aparticle diameter of 850 to 150 μm was 84% of the pulverization product.

Granulation Example 10

Water was added to the water-absorbent resin powder (A₂) underhigh-speed stirring in the mortar mixer of 5 liters (produced by NishiNihon Shikenki Seisakusho K.K.) in the same way as of GranulationExample 1 except that the temperature of 300 g of water to be added to200 g of the water-absorbent resin powder (A₂) was 20° C.

It took about 40 seconds to mix the water-absorbent resin powder (A₂)with water, and a huge united lumpy gel was formed, so nowater-absorbent resin granule in pieces was obtained.

Production Example 2-1

For Water-Absorbent Resin Powder

A reaction solution was prepared by dissolving polyethylene glycoldiacrylate (average molecular weight: 478) of 0.05 mol % as an internalcrosslinking agent into a 33 wt % aqueous solution of sodium acrylate(hydrophilic monomer) with a neutralization ratio of 75 mol %. Next, anitrogen gas was introduced into this reaction solution to decrease theamount of dissolved oxygen therein to not more than 0.1 ppm. Then, thereaction solution was supplied into a reaction vessel as prepared bycapping a stainless-steel-made double-arm type kneader having two sigmatype wings and a jacket. The reaction solution was adjusted to 20° C.,and the internal atmosphere of the reaction vessel was replaced with anitrogen gas. Subsequently, while stirring the reaction solution, sodiumpersulfate and L-ascorbic acid were added thereto as polymerizationinitiators in ratios of 0.14 g/mol and 0.008 g/mol, respectively,relative to the sodium acrylate.

As a result, 1 minute after the addition of the polymerizationinitiators, a polymerization reaction got started and, 16 minutes after,the reaction solution reached the peak temperature of 83° C., when theresultant hydrogel polymer was a finely particulated one with a size ofabout 5 mm. Then, the stirring was further continued, and the reactionhad been finished 60 minutes after the initiation of the polymerization,and the resultant finely particulated hydrogel polymer was thenseparated.

This finely-particulated hydrogel polymer was spread on a wire net andthen dried at 160° C. with a hot-air dryer for 65 minutes. Then, theresultant dried product was pulverized with a roll granulator (made byNippon Granulator K.K.), and then classified with a sieve of the meshsize of 500 μm, and further the residue of the dried product on thesieve was pulverized and classified again, thus obtaining an irregularpulverized water-absorbent resin powder (C) with an average particlediameter of 300 μm and a particle diameter distribution where theproportion of particles with a particle diameter of not larger than 105μm was 15% by weight.

Then, the resultant water-absorbent resin powder (C) was classified witha sieve of the mesh size of 105 μm, thus obtaining a water-absorbentresin powder (C₁) with a particle diameter of 105 to 500 μm, but notincluding 105 μm, and a water-absorbent resin powder (C₂) with aparticle diameter of not larger than 105 μtm.

Production Example 2-2

For Water-absorbent Resin Powder

A reaction solution was prepared by dissolving polyethylene glycoldiacrylate (average molecular weight: 478) of 0.04 mol % as an internalcrosslinking agent into a 35 wt % aqueous solution of sodium acrylate(hydrophilic monomer) with a neutralization ratio of 65 mol %. Next, anitrogen gas was introduced into this reaction solution to decrease theamount of dissolved oxygen therein to not more than 0.1 ppm.

Next, the reaction solution was injected in a thickness of 23 mm into astainless butt of which the inner surface was coated with Teflon. Then,the upper portion of this stainless butt was sealed with anacrylic-resin-made cap having a nitrogen-introducing inlet, an exhaustventage, and a polymerization-initiator-projecting inlet. Next, thisstainless butt was immersed into a water bath of 18° C. to adjust thetemperature of the reaction solution to 18° C. while introducing anitrogen gas into the reaction solution to decrease the amount ofdissolved oxygen therein to not more than 0.5 ppm. Subsequently, V-50(azo type polymerization initiator made by Wako Pure ChemicalIndustries, Ltd.), L-ascorbic acid, and hydrogen peroxide were added aspolymerization initiators to the reaction solution in ratios of 0.02g/mol, 0.0018 g/mol, and 0.0014 g/mol, respectively, relative to thesodium acrylate, and they were mixed sufficiently.

As a result, 1 minute after the addition of the polymerizationinitiators, a polymerization reaction got started. After confirming thepolymerization initiation, the above stainless butt was immersed into awater bath of 10° C. up to the height of 10 mm from the bottom of thestainless butt. Twelve minutes after the addition of the polymerizationinitiators, the reaction solution reached the peak temperature (82° C.).Then, the water bath of 10° C. was replaced with a water bath of 60° C.,in which the stainless butt was held for 60 minutes to finish thereaction.

Then, the resultant hydrogel polymer was separated from the stainlessbutt and then pulverized with a meat chopper having dice of 9.5 mm indiameter (No. 32 model chopper made by Hiraga Kosakusho K.K.) and thendried at 160° C. for 65 minutes. Then, the resultant dried product waspulverized with a roll granulator (Nippon Granulator K.K.), and thenclassified with a sieve of the mesh size of 500 μm, and further theresidue of the dried product on the sieve was pulverized and classifiedagain, thus obtaining an irregular pulverized water-absorbent resinpowder (D) with an average particle diameter of 280 μm and a particlediameter distribution where the proportion of particles with a particlediameter of not larger than 105 μm was 18% by weight.

Subsequently, an aqueous solution of surface-crosslinking agents,comprising 0.03 parts by weight of ethylene glycol diglycidyl ether, 1part by weight of propylene glycol, 3 parts by weight of water, and 1part by weight of isopropanol, was mixed with 100 parts by weight of theabove water-absorbent resin powder (D). The resultant mixture was heatedat 195° C. for 40 minutes, thus obtaining a surface-crosslinkedwater-absorbent resin powder (D′).

Then, the resultant water-absorbent resin powder (D′) was classifiedagain with a sieve of the mesh size of 850 μm and a sieve of the meshsize of 105 μm, thus obtaining a water-absorbent resin powder (D′₁) witha particle diameter of 105 to 850 μm, but not including 105 μm, and awater-absorbent resin powder (D′₂) with a particle diameter of notlarger than 105 μm.

Next, the water-absorbent resin powders with a particle diameter of notlarger than 105 μm, as obtained in the above production examples, weregranulated as follows:

EXAMPLE 2-1

Air was supplied into the continuous extrusion mixer 1 of FIGS. 4 and 5from one end portion thereof, namely, from the supplying-inlet 3 as madeat the left end in FIG. 4, to keep the reduced pressure inside thecasing 2 of the continuous extrusion mixer 1 not higher than 5 mmH₂O,while water as preheated to 80° C. was supplied at a rate of 165 kg/hrfrom the supplying-inlet 4 as made at a distance of 140 mm from the leftend of the casing 2 wherein the entire length of the rotary shaft 11present in the casing 2 of the continuous extrusion mixer 1 was 475 mm.

On the other hand, the water-absorbent resin powder (C₂), as obtained inProduction Example 2-1, was supplied with a proportioning supply machine(made by Accu-Rate Inc.) at a rate of 110 kg/hr into the continuousextrusion mixer 1 from the supplying-inlet 8 as made at a distance of228 mm from the left end of the rotary shaft 11 in the casing 2 (about52% downstream of the discharge outlet 10 wherein the entire length ofthe rotary shaft 11 was 100% ), and the stirring-members 12 were rotatedat 1,000 rpm, thus continuously mixing the water-absorbent resin powder(C₂) and water.

As a result, a particulate hydrogel granule (1) with a particle diameterof 1 to 5 mm was continuously obtained from the discharge outlet 10 asmade at the right end of the continuous extrusion mixer 1. At each timeof (1) 10 minutes, (2) 30 minutes, and (3) 60 minutes after the mixinginitiation, the mixing was stopped to measure the weight of thecontinuous extrusion mixer 1 to evaluate the amount of adhesivematerials as adhered to inside the continuous extrusion mixer 1 alongwith such adhered state. As a result, at any time of (1) to (3) above,it was seen that only a small amount of a mixture of water and thewater-absorbent resin powder (C₂) adhered mainly to the periphery of thesupplying-inlet 8 in the continuous extrusion mixer 1, but that did notinfluence the mixing (stirring). In addition, at any time of (1) to (3)above, the weight of the above adhesive materials was in the range of440 to 460 g.

Then, the resultant hydrogel granule (1) was spread on a wire net andthen dried at 160° C. with a hot-air dryer for 65 minutes. Then, theresultant dried product was pulverized with a roller mill (made by MeijiMachine K.K.), and then classified with a sieve of the mesh size of 500μm, and further the residue of the dried product on the sieve waspulverized and classified again in the same way as of Production Example2-1, thus obtaining an irregular pulverized water-absorbent resin powder(1) with an average particle diameter of 300 μm and a particle diameterdistribution where the proportion of particles with a particle diameterof not larger than 105 μm was 18% by weight, from which it would beunderstood that the above hydrogel granule (1) was a granule with almostas high a strength as a primary particle.

EXAMPLE 2-2

The water-absorbent resin powder (C₂) and water were continuously mixedin the same way as of Example 2-1 except that the water-absorbent resinpowder (C₂) was supplied into the continuous extrusion mixer 1 from thesupplying-inlet 9 as made at a distance of 317 mm from the left end ofthe rotary shaft 11 in the casing 2 (about 33% downstream of thedischarge outlet 10 wherein the entire length of the rotary shaft 11 was100% ).

As a result, a particulate hydrogel granule (2) with a particle diameterof 1 to 5 mm was continuously obtained from the discharge outlet 10 ofthe continuous extrusion mixer 1. Incidentally, the hydrogel granule (2)contained a small amount of fisheye-like water-absorbent resin powder(about 3% by weight) immediately after discharged from the dischargeoutlet 10, but became uniform soon.

In addition, an evaluation was made about the amount of adhesivematerials as adhered to inside the continuous extrusion mixer 1 alongwith such adhered state in the same way as of Example 2-1. As a result,at any time, it was seen that only a small amount of a mixture of waterand the water-absorbent resin powder (C₂) adhered mainly to theperiphery of the supplying-inlet 9 in the continuous extrusion mixer 1,but that did not influence the mixing (stirring). In addition, at anytime, the weight of the above adhesive materials was in the range of 450to 470 g.

Then, the resultant hydrogel granule (2) was dried, pulverized, andclassified in the same way as of Example 2-1, thus obtaining anirregular pulverized water-absorbent resin powder (2) with an averageparticle diameter of 300 μm and a particle diameter distribution wherethe proportion of particles with a particle diameter of not larger than105 μm was 19% by weight, from which it would be understood that theabove hydrogel granule (2) was a granule with almost as high a strengthas a primary particle.

EXAMPLE 2-3

The water-absorbent resin powder (D′₂) and water were continuously mixedin the same way as of Example 2-1 except that the water-absorbent resinpowder (C₂) was replaced with the water-absorbent resin powder (D′₂) asobtained in Production Example 2-2.

As a result, a particulate hydrogel granule (3) with a particle diameterof 1 to 5 mm was continuously obtained from the discharge outlet 10 ofthe continuous extrusion mixer 1. An evaluation was made about theamount of adhesive materials as adhered to inside the continuousextrusion mixer 1 along with such adhered state in the same way as ofExample 2-1. As a result, at any time, it was seen that only a smallamount of a mixture of water and the water-absorbent resin powder (D′₂)adhered mainly to the periphery of the supplying-inlet 8 in thecontinuous extrusion mixer 1, but that did not influence the mixing(stirring). In addition, at any time, the weight of the above adhesivematerials was in the range of 400 to 430 g.

Then, the resultant hydrogel granule (3) was dried, pulverized, andclassified in the same way as of Production Example 2-2, thus obtainingan irregular pulverized water-absorbent resin powder (3) with an averageparticle diameter of 280 μm and a particle diameter distribution wherethe proportion of particles with a particle diameter of not larger than105 μm was 20% by weight, from which it would be understood that theabove hydrogel granule (3) was a granule with almost as high a strengthas a primary particle.

EXAMPLE 2-4

The water-absorbent resin powder (C₂) and water were continuously mixedin the same way as of Example 2-1 except that the supplying-rate ofwater was changed from 165 kg/hr to 260 kg/hr.

As a result, a particulate hydrogel granule (4) with a particle diameterof 1 to 5 mm was continuously obtained from the discharge outlet 10 ofthe continuous extrusion mixer 1. An evaluation was made about theamount of adhesive materials as adhered to inside the continuousextrusion mixer 1 along with such adhered state in the same way as ofExample 2-1. As a result, at any time, it was seen that only a smallamount of a mixture of water and the water-absorbent resin powder (C₂)adhered mainly to the periphery of the supplying-inlet 8 in thecontinuous extrusion mixer 1, but that did not influence the mixing(stirring). In addition, at any time, the weight of the above adhesivematerials was in the range of 250 to 320 g.

Then, the resultant hydrogel granule (4) was dried, pulverized, andclassified in the same way as of Example 2-1, thus obtaining anirregular pulverized water-absorbent resin powder (4) with an averageparticle diameter of 300 μm and a particle diameter distribution wherethe proportion of particles with a particle diameter of not larger than105 μm was 15% by weight, from which it would be understood that theabove hydrogel granule (4) was a granule with almost as high a strengthas a primary particle.

Comparative Example 2-1

The water-absorbent resin powder (C₂) and water were continuously mixedin the same way as of Example 2-1 except that the water-absorbent resinpowder (C₂) was supplied into the continuous extrusion mixer 1 from thesupplying-inlet 3 as made at the left end in the casing 2, in otherwords, the water-absorbent resin powder (C₂) was supplied upstream ofwater.

As a result, a particulate comparative hydrogel granule (1) with aparticle diameter of 1 to 10 mm was obtained from the discharge outlet10 of the continuous extrusion mixer 1. The comparative hydrogel granule(1) contained a fisheye-like water-absorbent resin powder of about 15%by weight. An evaluation was made about the amount of adhesive materialsas adhered to inside the continuous extrusion mixer 1 along with suchadhered state in the same way as of Example 2-1. As a result, in 10minutes after the mixing initiation, a large amount of a mixture ofwater and the water-absorbent resin powder (C₂) adhered mainly to theabove supplying-inlet 3 and the stirring-members 12 . . . as locatedbetween the supplying-inlets 3 and 4, so the mixing became difficult tocontinue, when the weight of the above adhesive materials was 1,520 g.

Then, the resultant comparative hydrogel granule (1) was dried,pulverized, and classified in the same way as of Example 2-1, thusobtaining an irregular pulverized comparative water-absorbent resinpowder (1) with a particle diameter distribution where the proportion ofparticles with a particle diameter of not larger than 105 μm was 25% byweight.

Comparative Example 2-2

The water-absorbent resin powder (C₂) and water were continuously mixedin the same way as of Example 2-1 except that the water-absorbent resinpowder (C₂) was supplied into the continuous extrusion mixer 1 from thesupplying-inlet 7 as was made at a distance of 140 mm from the left endof the rotary shaft 11 in the casing 2 and was different from thesupplying-inlet 4, in other words, the water-absorbent resin powder (C₂)and water were supplied from the same position, but separately.

As a result, a particulate comparative hydrogel granule (2) with aparticle diameter of 1 to 10 mm was obtained from the discharge outlet10 of the continuous extrusion mixer 1. The comparative hydrogel granule(2) contained a fisheye-like water-absorbent resin powder of about 10%by weight. An evaluation was made about the amount of adhesive materialsas adhered to inside the continuous extrusion mixer 1 along with suchadhered state in the same way as of Example 2-1. As a result, a mixtureof water and the water-absorbent resin powder (C₂) adhered mainly to theperiphery of the supplying-inlet 7 and to upstream thereof, namely, tothe stirring-members 12 . . . as located between the supplying-inlets 3and 7, in the continuous extrusion mixer 1, and therefore, 30 minutesafter the mixing initiation, the mixing became difficult to continue,when the weight of the above adhesive materials was 1,300 g.

Then, the resultant comparative hydrogel granule (2) was dried,pulverized, and classified in the same way as of Example 2-1, thusobtaining an irregular pulverized comparative water-absorbent resinpowder (2) with a particle diameter distribution where the proportion ofparticles with a particle diameter of not larger than 105 μm was 22% byweight.

EXAMPLE 2-5

The water-absorbent resin powder (C₂) and water were continuously mixedin the same way as of Example 2-1 except that the supplying-rate of thewater-absorbent resin powder (C₂) was changed from 110 kg/hr to 65kg/hr, and that the supplying-rate of water was changed from 165 kg/hrto 65 kg/hr, and that the temperature of water was changed from 80° C.to 70° C.

As a result, a particulate hydrogel granule (5) with a particle diameterof 1 to 5 mm was continuously obtained from the discharge outlet 10 ofthe continuous extrusion mixer 1. An evaluation was made about theamount of adhesive materials as adhered to inside the continuousextrusion mixer 1 along with such adhered state in the same way as ofExample 2-1. As a result, at any time, it was seen that only a smallamount of a mixture of water and the water-absorbent resin powder (C₂)adhered mainly to the periphery of the supplying-inlet 8 in thecontinuous extrusion mixer 1, but that did not influence the mixing(stirring). In addition, at any time, the weight of the above adhesivematerials was in the range of 500 to 550 g.

Then, the resultant hydrogel granule (5) was dried, pulverized, andclassified in the same way as of Example 2-1, thus obtaining anirregular pulverized water-absorbent resin powder (5) with an averageparticle diameter of 310 μm and a particle diameter distribution wherethe proportion of particles with a particle diameter of not larger than105 μm was 20% by weight, from which it would be understood that theabove hydrogel granule (5) was a granule with almost as high a strengthas a primary particle.

EXAMPLE 2-6

The water-absorbent resin powder (D′₂) and water were continuously mixedin the same way as of Example 2-3 except that the supplying-rate of thewater-absorbent resin powder (D′₂) was changed from 110 kg/hr to 66kg/hr, and that the supplying-rate of water was changed from 165 kg/hrto 54 kg/hr, and that the temperature of water was changed from 80° C.to 90° C., and that the revolution number of the stirring-members 12 waschanged from 1,000 rpm to 1,500 rpm.

As a result, a particulate hydrogel granule (6) with a particle diameterof 1 to 5 mm was continuously obtained from the discharge outlet 10 ofthe continuous extrusion mixer 1. An evaluation was made about theamount of adhesive materials as adhered to inside the continuousextrusion mixer 1 along with such adhered state in the same way as ofExample 2-1. As a result, at any time, it was seen that only a smallamount of a mixture of water and the water-absorbent resin powder (D′₂)adhered mainly to the periphery of the supplying-inlet 8 in thecontinuous extrusion mixer 1, but that did not influence the mixing(stirring). In addition, at any time, the weight of the above adhesivematerials was in the range of 600 to 650 g.

Then, the resultant hydrogel granule (6) was dried, pulverized, andclassified in the same way as of Production Example 2-2, thus obtainingan irregular pulverized water-absorbent resin powder (6) with an averageparticle diameter of 290 μm and a particle diameter distribution wherethe proportion of particles with a particle diameter of not larger than105 μm was 21% by weight, from which it would be understood that theabove hydrogel granule (6) was a granule with almost as high a strengthas a primary particle.

From the results of Examples 2-1 to 2-6 and Comparative Examples 2-1 to2-2, it would be understood that if the water-absorbent resin powder issupplied downstream of water, then a stable mixing can be carried out,for a long term, and further the amount of the formation of the finepowder, as included in the water-absorbent resin powder as obtained bydrying and pulverizing the resultant hydrogel granule, can be reduced(in other words, the granulation strength can be enhanced still more),when compared with the case where the water-absorbent resin powder issupplied upstream of water or where the supplying-position of thewater-absorbent resin powder and that of water are made even.

In addition, as to Comparative Examples 2-1 and 2-2, the residence timeof the water-absorbent resin powder (C₂) in the continuous extrusionmixer 1 was longer than that in Example 2-1, whereas fisheyes wereincluded in the resultant hydrogel granule. From this result, it can beguessed that when the water-absorbent resin powder is supplied upstreamof water or at the same position as water, partial unevenness occurs inthe contact between the water-absorbent resin powder and water, and thatthis is a factor in failure to make a stable mixing for a long term.

Various details of the invention may be changed without departing fromits spirit not its scope. Furthermore, the foregoing description of thepreferred embodiments according to the present invention is provided forthe purpose of illustration only, and not for the purpose of limitingthe invention as defined by the appended claims and their equivalents.

What is claimed is:
 1. A process for granulating particles to producegranulated particles, with the granulated particles being greater insize than the particles, with the particles being water-absorbent resinparticles, with the process comprising the steps of: a) providing areceptacle; b) providing a supply of the particles that have an averageparticle diameter of 10 to 150 μm; c) providing a supply of aqueousliquid; d) controlling the aqueous liquid to a temperature of 40° C. tothe boiling point of the aqueous liquid prior to supplying the aqueousliquid to the receptacle; and e) mixing the aqueous liquid and particlesin the receptacle at a high speed, with the step of mixing comprisingthe steps of: i) introducing the aqueous liquid to the receptacle whilethe aqueous liquid has a temperature of 40° C. to the boiling point ofthe aqueous liquid; ii) introducing the particles to the receptacle;iii) bringing the aqueous liquid and particles into contact with eachother while the aqueous liquid has a temperature of 40° C. to theboiling point of the aqueous liquid and mixing the aqueous liquid andparticles in the receptacle while the aqueous liquid has a temperatureof 40° C. to the boiling point of the aqueous liquid; iv) mixing theaqueous liquid and particles in the receptacle for at longest threeminutes; and v) obtaining granulate particles not larger than 20 mm indiameter.
 2. The process for granulating particles according to claim 1,wherein the steps of introducing particles and aqueous liquid to thereceptacle comprises the step of adding 100 parts by weight of theparticles and 80 to 280 parts by weight of the aqueous liquid.
 3. Theprocess for granulating particles according to claim 1, wherein the stepof controlling the aqueous liquid comprises the step of controlling theaqueous liquid to a temperature of 50 to 100° C., and wherein the stepof introducing the aqueous liquid to the receptacle comprises the stepof introducing the aqueous liquid to the receptacle at a temperature of50 to 100° C.
 4. The process for granulating particles according toclaim 1, and further comprising the step of drying a resultant mixturecomprising particles and aqueous liquid, with the step of drying theresultant mixture comprising the step of controlling the resultantmixture to a temperature of 100 to 250° C.
 5. The process forgranulating particles according to claim 1, and further comprising thesteps of: a) controlling the particles to a temperature of 40 to 100° C.prior to supplying the particles to the receptacle; and b) introducingthe particles to the receptacle while the particles have a temperatureof 40 to 100° C.
 6. The process for granulating particles according toclaim 1, wherein the step of controlling the aqueous liquid to atemperature of 40° C. to the boiling point of the aqueous liquidcomprises the step of preheating the aqueous liquid to a temperature of40° C. to the boiling point of the aqueous liquid.
 7. The process forgranulating particles according to claim 1, wherein the boiling point ofthe aqueous liquid is 100° C.
 8. The process for granulating particlesaccording to claim 1, wherein the step of introducing particles to thereceptacle comprises the step of adding particles where at least 70% byweight of the particles have a particle diameter of not greater than 150μm.
 9. A process for granulating fine particles to produce granulatedparticles directly from the fine particles without producing a unitedkneaded mass, with the granulated particles being greater in size thanthe fine particles, with the fine particles being water-absorbent resinparticles, with the process comprising the steps of: a) providing areceptacle; b) providing a supply of the fine particles; c) providing asupply of aqueous liquid; d) controlling the aqueous liquid to atemperature of 40° C. to the boiling point of the aqueous liquid priorto supplying the aqueous liquid to the receptacle; e) mixing the aqueousliquid and fine particles in the receptacle at a high speed, with thestep of mixing comprising the steps of: i) introducing the aqueousliquid to the receptacle while the aqueous liquid has a temperature of40° C. to the boiling point of the aqueous liquid; ii) introducing thefine particles to the receptacle; iii) bringing the aqueous liquid andparticles into contact with each other while the aqueous liquid has atemperature of 40° C. to the boiling point of the aqueous liquid andmixing the aqueous liquid and fine particles with each other in thereceptacle at a high speed; and iv) avoiding a united kneaded mass ofparticles during the step of mixing; v) such that the fine particles aregranulated without kneading to produce the granulated particles directlyfrom the fine particles.
 10. The process for granulating particlesaccording to claim 9, wherein the steps of introducing particles andaqueous liquid to the receptacle comprises the step of adding 100 partsby weight of the particles and 80 to 280 parts by weight of the aqueousliquid.
 11. The process for granulating particles according to claim 9,wherein the step of controlling the aqueous liquid comprises the step ofcontrolling the aqueous liquid to a temperature of 50 to 100° C., andwherein the step of introducing the aqueous liquid to the receptaclecomprises the step of introducing the aqueous liquid to the receptacleat a temperature of 50 to 100° C.
 12. The process for granulatingparticles according to claim 9, and further comprising the step ofdrying a resultant mixture comprising particles and aqueous liquid, withthe step of drying the resultant mixture comprising the step ofcontrolling the resultant mixture to a temperature of 100 to 250° C. 13.The process for granulating particles according to claim 9, and furthercomprising the steps of: a) controlling the particles to a temperatureof 40 to 100° C. prior to supplying the particles to the receptacle; andb) introducing the particles to the receptacle while the particles havea temperature of 40 to 100° C.
 14. The process for granulating particlesaccording to claim 9, wherein the step of controlling the aqueous liquidto a temperature of 40° C. to the boiling point of the aqueous liquidcomprises the step of preheating the aqueous liquid to a temperature of40° C. to the boiling point of the aqueous liquid.
 15. A process forgranulating fine particles to produce granulated particles directly fromthe fine particles without producing a united kneaded mass, with thegranulated particles being greater in size than the fine particles, withthe fine particles being water-absorbent resin particles, with theprocess comprising the steps of: a) providing a receptacle; b) providinga supply of the fine particles that have an average particle diameter of10 to 150 μm; c) providing a supply of aqueous liquid; d) controllingthe aqueous liquid to a temperature of 40° C. to the boiling point ofthe aqueous liquid prior to supplying the aqueous liquid to thereceptacle; e) mixing the aqueous liquid and fine particles in thereceptacle at a high speed, with the step of mixing comprising the stepsof: i) introducing the aqueous liquid to the receptacle while theaqueous liquid has a temperature of 40° C. to the boiling point of theaqueous liquid; ii) introducing the fine particles to the receptacle;iii) bringing the aqueous liquid and particles into contact with eachother while the aqueous liquid has a temperature of 40° C. to theboiling point of the aqueous liquid and mixing the aqueous liquid andfine particles in the receptacle while the aqueous liquid has atemperature of 40° C. to the boiling point of the aqueous liquid; iv)mixing the aqueous liquid and particle in the receptacle for at longestthree minutes; v) avoiding a united kneaded mass of particles during thestep of mixing; vi) obtaining granulate particles not larger than 20 mmin diameter such that the fine particles are granulated without kneadingto produce the granulated particles directly from the fine particles.16. The process for granulating particles according to claim 15, whereinthe steps of introducing particles and aqueous liquid to the receptaclecomprises the step of adding 100 parts by weight of the particles and 80to 280 parts by weight of the aqueous liquid.
 17. The process forgranulating particles according to claim 15, wherein the step ofcontrolling the aqueous liquid comprises the step of controlling theaqueous liquid to a temperature of 50 to 100° C., and wherein the stepof introducing the aqueous liquid to the receptacle comprises the stepof introducing the aqueous liquid to the receptacle at a temperature of50 to 100° C.
 18. The process for granulating particles according toclaim 15, and further comprising the step of drying a resultant mixturecomprising particles and aqueous liquid, with the step of drying theresultant mixture comprising the step of controlling the resultantmixture to a temperature of 100 to 250° C.
 19. The process forgranulating particles according to claim 15, and further comprising thesteps of: a) controlling the particles to a temperature of 40 to 100° C.prior to supplying the particles to the receptacle; and b) introducingthe particles to the receptacle while the particles have a temperatureof 40 to 100° C.
 20. The process for granulating particles according toclaim 15, wherein the step of controlling the aqueous liquid to atemperature of 40° C. to the boiling point of the aqueous liquidcomprises the step of preheating the aqueous liquid to a temperature of40° C. to the boiling point of the aqueous liquid.
 21. A process forgranulating water-absorbent resin particles to produce granulatedparticles, with the granulated particles being greater in size than thewater-absorbent resin particles, with the process comprising the stepsof: a) providing a supply of the water-absorbent resin particles thathave an average particle diameter of 10 to 150 μm; b) providing a supplyof aqueous liquid; c) controlling the aqueous liquid to a temperature of40° C. to the boiling point of the aqueous liquid prior to contactingthe aqueous liquid with the water-absorbent resin particles; d) bringingthe aqueous liquid and water-absorbent resin particles into contact witheach other while the aqueous liquid has a temperature of 40° C. to theboiling point of the aqueous liquid and mixing the aqueous liquid andwater-absorbent resin particles together at a high speed for at longestthree minutes; and then e) obtaining granulate particles not larger than20 mm in diameter.